I figure Stark, Rogers, Rhodes and Thor (or Loki) are all going to bite it in one of the two movies, most of them because their contracts are up (RDJ is on movie-to-movie but he's been making more and more noise about being too old for this). Maybe an ancillary character like Drax or, I don't know, Wong.
The easy answer, comics-wise, would be that they replace Iron Man with Rescue (Pepper Potts), but I think that's unlikely given how much Gwyneth Paltrow has trashed her experiences with Marvel Studios. They might actually give Iron Man a rest for a while before doing something like putting Peter Parker in the suit.
I pretty much expect everyone to die - or at least almost everyone. But most of them will be brought back, of course. Rogers will be gone for good. Stark will probably be dead, but might return on a case by case basis as the Stark AI. Thor seems a probable candidate, cause what else is there for him to do with Asgard gone and his trilogy finished? Hawkeye could go several different ways - death, actual retirement, or taking a larger leadership role in the team. Black Widow shouldn't die permanently, as Feige has claimed his intention to do a BW movie in phase 4. Rhodes... eh, he could easily die, but I don't think his death serves much purpose. Not many people are going to wail at the thought of losing him. And it could bring bad publicity with killing off one of the few non-white heroes they've got. Especially after the blatant death bating of civil war. It'd be Glen from Walking Dead all over again.
The rest should be safe as the 'newer' characters who ought to be sticking around to carry the MCU forward. Although, Vision worries me a bit. I really don't want him to die, but the mind stone thing seems pretty major.
And, yeah, ancillary characters may be a possibility as well, though personally I'd go for Winter Soldier (since Cap's mantle can be carried by Falcon and Stan's 9 movie contract would make his death an actual surprise).
The big question mark for me in this regard is the Guardians. On the one hand, it'd be weird if only Avengers died. On the other hand, how does Guardians 3 deal with the fact that one or more of the teammates died in some other movie? That seems to be pushing the limits of the shared universe formula. And if they do want to, who goes? Star-lord and Gamora are the main heroes of the team. Rocket and Groot are the fan favorites. Mantis would be a bit of a cop-out who people don't yet care that much about. Which means Drax and Nebula. I guess that makes sense since both of them actively want to kill Thanos.
Plus, nobody who dies in the MCU ever really stays dead.
Except for all the people who have, right?
I wonder if they could get RDJ back just as a CGI hologram/AI for a War Machine & Riri Williams Ironman torch-passing. A day in a recording studio for however many millions might be a tempting offer.
Teenage genius girl reverse engineers a suit of the now dead Tony, stumbles on some hidden protocols where Tony cleverly hid an AI of himself knowing that eventually someone would be smart enough to find it and that person would be a worthy heir to both his lab and mantle. It's not a fresh take but its plenty enough for Marvel to make something great, and it would be nice to see a more gender-balanced lineup for the next phase/era of films.
This is exactly what I hope we see for any future IM movie.