My predictions for the fates of each character:
Uncle Sam: Going to die 100%. One of the biggest things he faces as a character is "being a soldier without a war to fight", so having him die in combat I think is a fitting end for the character. Plus, Evan's contract is up and he has expressed no interest in renewal.
Metal Man: Either going to die or retire. RDJ is acting like these are his last movies in the MCU, his contract is up, he's expensive, and it would be a perfect end for his character arc (especially retirement). Plus, his death could spark some interesting internal conflict with Peter and Spidey 2 could focus on him trying to cope and/or finding a new father figure/mentor (which could be a great set-up to introduce someone like Doc Oc or Green Goblin).
Point Break: Hemsworth's contract is up, but he's expressed interest in renewing it now that his character has developed into something interesting. Plus he just got a crap load of powers. I don't see him dying, but he could take on a lesser role in future movies. If he does, Vaklyrie will probably take over his mantle and rule New Asgard.
Hide the Cucumber: Ruffalo has 2 movies left on his contract after A4. He's going to live.
Thigh Chokehold: Black Widow has a solo movie coming up and she hasn't really gotten enough focus yet. I don't see her dying or retiring.
The True Strongest Avenger: Renner still has two movies on his contract after A4 iirc. Even if one of them is a potential BW/Hawkeye flashback movie, that still leaves one on his contract. He could die, but I doubt it.
Baby Robot Goku Jesus: Dead.
Birdman: Like Thor, Mackie's contract is up with A4. I'm feeling this is Falcon's last rodeo.
Aluminum Man: Really, the one I'm least sure on. I don't see him having a big role in future movies, so I'm going to guess either death or retirement.
Grower, Not a Shower: 0% chance of Ant-Man dying. 0. Z-E-R-O. He's one of the leaders of the next-gen of Marvel movies and is two movies into a trilogy now.
Arachnoboy: He gon get resurrected, then be an important figure coming up. Also he has a movie. So.
Mr. Doctor: C'mon. We all know he's gonna live.
Cat-Man: He's only used up like...two of his 9 lives. Plus he'll likely have a sequel.
Hermione Granger: Another one I'm not so sure on. Her trauma over Baby Robot Goku Jesus's death could make interesting fodder for a solo movie or to set her up as a future "I can't control my powers"/Jean Grey/Phoenix-type villain. Or she could just die.
Gonna Need That Arm: He has like 80,000,000 movies left on his contract. He probably won't take up the role of Cap like some people thing, but I do not see Bucky dying.
Chubby Chipmunk: Rocket ain't goin nowhere.
Space Man: Quill has a lot of unresolved character development that probably needs to happen. I don't see him dying.
The Invisible Man: Drax could die. Or we could have more character development from him trying to figure out what to do after Thanos bites the dus-um wow poor choice of words.
I Am Groot: I Am Groot.
Why Are There So Many Characters?: Mantis was just introduced. C'mon.
Green Giant Green Beans: I'd say she stays dead...until GotG Vol. 3.
Cyborg Amelia Pond: Either joins the Guardians or dies. I'm leaning towards the later.
Barney: Seriously? Thanos is the villain. He gon die.
Watch me be completely wrong.
Yeah, that's kind of what I was thinking. I'm also wondering whether A4 will somewhat follow the pattern of Return of the King, with the main action being over, and then an extended epilogue where the film just kind of ties up the loose ends and closes the book on a lot of characters we won't be seeing again.
I never realized how much I wanted this. It'd be such a nice way to end the movie.
If they don't kill him, they can easily have him head a new SHIELD or some equivalent and pass on the Iron Man persona to someone else. That way he can make appearances if necessary but won't be central to plots. They could also do this to Cap, but the MCU Cap doesn't really fit that.
I don't see them making Stark head of SHIELD. It'd really step on the toes of the television shows and rile up a bunch of fans (which they probs don't want). Plus, it'd make all that "end-game" foreshadowing moot and would hurt his character development imo.