With Avengers Infinity War just around the corner I thought it would be fun to speculate on what we might see in the film. There are no spoilers for AIW below--everything discussed is via the two trailers (one and two) and other promotional material/public comments (including the prelude comic). There are, however, spoilers for all previous MCU movies.
Most fans are aware of the comic inspirations for this story, which was first hinted at in The Avengers (2012). The comic inspiration begins with Thanos Quest (1990), whose story continues through The Infinity Crusade (1993); perhaps more important are the recent Thanos Rising and Infinity (2013) stories. The film isn't intended as a direct adaptation (Captain America: Civil War (2016) is a good template to keep in mind as that adaptation was also by the Russo's), but many elements are borrowed, particularly from the latter series. I won't go through the plots of each comic series (for those interested you can start here) or their many known connections to the films, as such explorations are plentiful online and I'm less interested in that then in speculating about the movie itself. I will point out that Thanos' children (other than Gamora and Nebula) are creations of the 2013 comic--known as the Black Order, they consist of Proxima Midnight, Cull Obsidian, Corvus Glaive, and Ebony Maw, and seem to feature prominently in the film.
The MCU-version of Thanos, like his comicbook counterpart, is seeking the six infinity stones to power the infinity gauntlet. His motivations for doing so are, however, very different. In the comics Thanos is in love with the manifestation of Death, so he seeks power and death to impress her. In the MCU he wants to bring balance to the universe (requiring the death of half the life in it--we don't know why he thinks this is necessary, although the second trailer suggests it has been his goal for quite some time)--this puts an end to theories that Hela would be a stand-in for Death. I don't think the movie-specifics of this idea were settled until recently--Kevin Feige clearly intended the MCU to culminate in a struggle against Thanos prior to his first appearance (2012), but given how much the comics written afterwards seem to have impacted things, screenwriters Christopher Markus and Stephen McFeely had fairly free reign when they wrote the screenplay (which was composed no later than 2016). I definitely think a more nuanced, sympathetic Thanos will appear on screen.
The only active role Thanos has had in the MCU prior to these films was in Guardians of the Galaxy (2014), but his role there was virtually identical to the one he provided in the background of The Avengers--motivating a subordinate antagonist (Ronan and Loki) to gather an infinity stone for him. In the post-credit scene of Avengers: Age of Ultron (2015) he declares that he'll gather the stones himself and that's the true beginning of these films.
The stones themselves have featured much more prominently than their seeker in the MCU:
-The Tesseract (space stone) is an important part of Captain America (2011), having been introduced in the post-credit scene of Thor (2011), and subsequently of importance in The Avengers before being stored on Asgard (Loki steals it in Thor: Ragnarok (2017), a supposition confirmed by the first trailer)
-The Mind Stone is given to Loki by Thanos in The Avengers, which is subsequently used by Hydra to create Scarlett Witch and Quicksilver in the post-credit scene of Captain America: Winter Soldier (2014); it gets retrieved and becomes part of Ultron's creation of Vision in Avengers: Age of Ultron--Vision later appears in Captain America: Civil War (2016) and the stone continues to be a part of him
-The Aether (reality stone) is at the core of the plot of Thor: The Dark World (2013), which when recovered is given to the Collector who, despite the explosion of his collection in Guardians of the Galaxy, presumably still has it
-The Orb (power stone) is the core plot element of Guardians of the Galaxy which, when resolved, is stored on Xandar
-The Eye of Agamotto (time stone) has been held by earth's sorcerers for millennia, appearing in Doctor Strange (2016) and remaining with them
-The Soul Stone's location is completely unknown and, other than its image being shown by the Collector to the Guardians, virtually nothing is known about its location
What's interesting is that in the seven films since Age of Ultron the stones have only been an important plot element in one, Doctor Strange. I'm not sure what to make of that decision (since they were vital in five of seven previously), but their primary story will be wrapped up by the next two installments of the Avengers.
The gauntlet itself has been on its own brief, somewhat bizarre journey within the MCU, showing up as an easter egg in the first Thor (2011)--its appearance retained as another easter egg for Thor: Ragnarok where it was confirmed as a fake. In Age of Ultron we see that Thanos already has the real gauntlet, but we won't see it in action until Infinity War.
With the background out of the way let's try to explore the story as presented in the trailers. We need to keep in mind that the film is intended to have a definitive ending, such that even though the fourth Avengers film is a sequel it's not meant to end on an unresolved cliffhanger.
Where We Begin
Many of the Avengers are on the run after the signing of the Sokovia Accords (Civil War)--Captain America, Black Widow, and Falcon are together and operating clandestinely as a team; Vision and Scarlett Witch are together apparently just as a couple (retired, perhaps); Hawkeye is retired and probably back with his family; Ant-Man is on his own (likely under a form of house arrest so that he can still see his daughter); Iron Man, War Machine, Spider-Man, and Doctor Strange are in New York; Bucky remains in Wakanda with Black Panther; the Guardians are together (including additions Mantis and Nebula); Thor and Hulk are on the Asgardian lifeboat which, at the end of Ragnarok, is confronted by Thanos' ship. Newcomers the Wasp and Captain Marvel won't appear until their own films (although post-credit appearances remain possible).
-Introduction to Thanos (the Russo's describe it as primarily his movie)--flashback to his origin and the death of his people, which serves as his call to action that informs his arc throughout this and the next film--likely followed by his successful attack on Xandar where he seizes the Orb (power stone)--the way the trailers are cut he acquires this stone first--it's unclear if the few Xandarians we were introduced in Guardians of the Galaxy will survive (Nova Prime and Dey), but I'd expect at least Glenn Close's character to be killed if she appears at all. It's possible this content occurs after the Asgardian lifeboat scene below.
-Thanos attacks the Asgardian lifeboat, defeating those on board; Loki uses the Tesseract (space stone) and his wit to save some of them (I don't believe Asgardian genocide will be on the table for this story as they are too integral to the future of the Thor franchise)--Thor and Hulk are sent away via the space stone (perhaps by Loki himself); I expect that the characters introduced in Ragnarok survive (Valkyrie, Korg, etc), but Heimdall might be killed (much as I'd rather keep Idris Elba); I've seen speculation that Loki will also die, but unless Tom Hiddleston is tired of playing the character I think that's very unlikely--he's an extremely popular character with plenty of untapped potential
-The Guardians crash into Thor in space; Gamora talks about Thanos with a flashback to when she was taken (the info dump augmented by Nebula and includes info about their other siblings, the Black Order); Thor, Rocket, and Groot head off to the dwarves (some think this will include Peter Dinklage's character, but it's just as likely he's providing the voice for one of the Black Order--this despite his notable failure as a voice actor in Destiny) and create his new weapon (presumed to be Stormbreaker--in the comics the weapon used by Beta Ray Bill)--the rest of the Guardians go to Knowhere (see below)
-Cull Obsidian and Ebony Maw arrive in New York to take the Eye of Agamotto (time stone) away from Doctor Strange--Wong is probably involved in this fight, but I suspect the villains are successful in taking it (which is why the ship Spider-Man see's is departing Earth and Iron Man has to race after him)--I'm assuming that they lose the time stone here because I think the fight in Wakanda is over the final infinity stone (either literally the last or the last other than the soul stone, but I expect the former), and because it gives the Black Order a "win" over the heroes, but this could happen later
-The Hulk crashes into the Sanctum in New York (Doctor Strange and Wong are there), which results in Iron Man showing up; Banner explains what Thanos is about--this could happen prior to the loss of the Eye of Agamotto, although I think the result will be the same
-Scarlett Witch and Vision are attacked by Corvus Glaive and Proxima Midnight in an attempt to take the mind stone--Captain America, Black Widow, and Falcon arrive and help defeat them, but Vision is badly wounded in the process--as a result, they will head to New York to meet with the other Avengers (likely after a call from Tony--see below); it's possible Tony's call is what has them arriving to save their friends
-Spider-Man see's one of Thanos' ships floating overhead and hurries to investigate--this occurs while Doctor Strange, Wong, Bruce Banner, and Iron Man are talking and Tony hurries out as the ship with Spider-Man begins accelerating into space--Iron Man brings him the iron spider suit so that Peter can survive in space
-The Guardians (minus Rocket and Groot) go to Knowhere to find the Collector and the Aether (reality stone); Thanos beats them there via the space stone, seizing the reality stone either before the Guardians or forcing them to flee (I fully expect the Collector to survive); when they fail they head to earth where two of the remaining infinity stones are
-Tony has called Cap (this might occur before the above action with Peter Parker) and most of the Avengers assemble at their HQ (absent Bucky, Thor, Ant-Man, possibly Hawkeye, and unofficial Avenger Black Panther); I believe the Guardians (minus the aforementioned) meet them here because this is where the decision to split the team occurs--Captain America, War Machine, Black Widow, Falcon, Scalett Witch, Vision, and Bruce Banner in the Hulkbuster go to Wakanda, while Iron Man, Doctor Strange, and Spider-Man join the remaining Guardians (Star-Lord, Gamora, Nebula, Mantis, and Drax) in space heading to Titan--Wong presumably remains behind to defend the Sanctum. I believe the journey to Titan is to save the soul stone (there's no other reason to go there with the information we have)
-Hawkeye is sent on a mission of his own and a theory I like is that he's seeking out his old S.H.I.E.L.D. buddy Carol Danvers (a theory I first heard here)--this might be where we see Ant-Man--presumably he'll join the team for the Wakanda fight at the end; alternatively Clint is specifically looking for Scott Lang; I believe Hawkeye's family (or at least his wife) could be a casualty, although in what context I'm not sure yet (there's no obvious reason for the Black Order to seek them out)--my speculation is purely based on how poorly they were received and how they function better as character motivation for Clint to return to being a hero (he's really had no arc in the MCU yet)
-Thor and crew successfully create Stormbreaker; the Russo's have said he has a lot of screen time so there may be more going on with this plot than is readily apparent
-Shuri attempts to heal Vision--it's not clear if she's trying to separate the mind stone from his body (I've seen conflicting speculation, but given that I don't think he's likely to die, I expect she's trying to extract it and will succeed--this would present interesting character development for what is a pretty one-note hero)
-The space team confront Thanos on what's assumed to be his home planet of Titan--my guess (as mentioned above) is that they are fighting over the soul stone and it appears as though they fail (why the stone is there I'm not sure, but there isn't any other obvious reason for them to be fighting him there); it's possible Thor arrives dramatically to prevent them from dying at the end of this battle (see below)
-The Battle of Wakanda: my guess is the team will fail until the cavalry arrives (the heroes who were in space) to avoid total annihilation, but it remains to be seen how Thanos can both get the sixth stone and the heroes still get a win; it's likely that they fail and the fourth movie is about preventing Thanos from following through on his plan (this would give dramatic license for Captain Marvel's arrival to help); I wasn't sure Ant-Man and the Wasp would have any connection AIW, but I've heard recently that it will impact the fourth movie
I don't think any of the infinity stones will be held in reserve for the fourth movie, such that in the next installment it will be about defeating Thanos with the infinity gauntlet. With that said I don't don't think the movie will end on a downer and the heroes will have achieved some sort of victory (even if it's a Pyrrhic one). I suspect the Russo's are aiming for an Empire Strikes Back feel where there's a sense of hope after failure (I'd guess that Thanos gets the stones, but his children and army are defeated).
An interesting theory is going around (eg here and here) that the movie will end with all the core Avengers (or at least some of them) dying with Thanos winning--Doctor Strange then sends heroes back in time to save them. There are reasons for suspecting time travel isn't an element in the fourth film, but it's really hard to judge the theories until we see how Infinity War unfolds.
As for deaths, I don't think we'll see more than one major character die in this movie (and even then it might not be permanent). Secondary and tertiary characters will fall however (much as they did in Thor: Ragnarok)--as mentioned above I'd guess Hawkeye's family (if not this movie, the next), Heimdall, Nova Prime, and perhaps a few others.
The Fourth Avengers Film
I've seen a few theories about this leading to a new timeline/partial universe reset to make room for the Fox properties (the mutants in particular), but I think this is very unlikely (the idea would remove the sense of consequence from previous stories). The writers of the fourth film have been keeping an eye on fan theories and said that none were right (it's difficult to establish when this interview took place--the report says "on the set" of Infinity War, which wrapped shooting in July of 2017).
In the untitled fourth Avengers movie (quite possibly Infinity Gauntlet assuming Zoe Saldana's slip of the tongue a year ago can be taken literally), however, I think we'll see casualties among the lead heroes. Briefly: Hawkeye (always a peripheral character) is commonly assumed to be among the dead; Nebula, whose only remaining character arc is killing Thanos, seems like an obvious choice as well (Gamora is also an option in this respect, but I suspect James Gunn wants to hang on to most of the Guardians for the third and final film of that iteration).
In addition to those two characters I think there are a number of others who might go simply due to their actors aging out: War Machine (Don Cheadle will be pushing 55 when the film comes out), even if it seems a bit redundant after he recovers from the spine injury from Civil War; Hulk (Mark Ruffalo will be pushing 52-- I'm not sure what will be left to do with his version of the character); and Iron Man--a suitably epic ending in his ninth major film (he'd be 54 at release; if he dies I think Robert Downey Jr. will remain as a voice however--his consciousness downloaded--echoing something that happened in the comics). If Banner goes we might see either the Amadeus Cho version of the Hulk or perhaps She-Hulk (problems with Universal's involvement seem negligible so long as they don't get solo films); there's also a chance Shuri (or someone else) will pick up the mantel of Iron Man as Iron Heart (even though the comicbook version of that character is enormously unpopular, the MCU could do a much better job with her).
Originally I thought Captain America would die (Chris Evans is always talking about potentially leaving--see below), but there's no point in giving Bucky the new identity of White Wolf (Black Panther post-credit scene) if he isn't going to wear that for awhile and although Falcon could also take up the mantel I don't think we'll see that yet. Chris Evans will only be 38 once the film is out and is popular enough in the role that Disney will likely pay him whatever he wants to stay. A year ago I would have put Thor on the death list, but Chris Hemsworth had so much fun making Ragnarok, whose new direction fits the cosmic direction of the MCU, that I expect him to stick around.
The true crux of it all is we're unlikely to see many deaths at all--they are extremely rare in the comics (and almost never permanent) and as a massively successful franchise there's little impetus for the MCU to suddenly start killing off characters unless there are external reasons to do so.
Going back to Chris Evans for a moment: a New York Times article about him has been used as the source for innumerable articles and videos claiming he's done as Captain America. Here is the actual quote:
For now, he has no plans to return to the franchise (“You want to get off the train before they push you off,” he said), and expects that planned reshoots in the fall  will mark the end of his tenure in the familiar red, white and blue super suit.This is Reggie Ugwu (the reporter) providing statements about Evans' plans that suggest (and only suggest) finality. The predictable hoard of clickbait articles and videos echoing this have been so ridiculous that others have pointed out the obvious issue with that conclusion. This is also not dissimilar to the difficult contract negotiations with Robert Downey Jr. after Iron Man 3 (whose ending was made in such a way that if he didn't re-sign there would be a reason for him not to appear in Age of Ultron). This, for different reasons, is also worth keeping in mind in regards to Benedict Cumberbatch's comments that there are no plans for Doctor Strange 2--I believe in his case Cumberbatch wants to maintain the illusion that Strange could die in the Avengers films, because if he admits to a sequel fans will know he has the plot armour of a sequel.
Those are my thoughts. I'm sure I'll be hilariously wrong about some details, but it's a lot of fun to speculate.
This article is written by Peter Levi (@eyeonthesens)