1) I think you all are drastically overestimating how many people will die in this. Not only is it based on comics where nobody ever truly dies, but this is also the universe of infinite fake-outs.
For example: Coulson died and went to TV, Bucky died but not really, Pepper fell to her death but not really, Nick Fury was killed but not really, Loki died but not really, Loki died again but not really, etc. etc.
The only real deaths so far have been Quicksilver and Yondu. Decrepit Peggy Carter doesn't count.
2) One death is dramatic. Two is heavy. But y'all are talking about 4-8 major fan- (and child-) favorites in a span of about 4 hours.
3) If you kill off a character, you risk losing out on some merchandise potential. Also, crossover potential going forward. Who has stories left to tell?
4) There are ways to write off a character besides killing them.
5) As long as the time stone is in play, any death can be reversed. I suppose what's that's nullified somehow, the option isn't there, but I'll say this: in the 90s Infinity Gauntlet comics, nearly EVERYONE dies, but by the end of the series, everyone is alive again.
Here's my vote:
Captain America - dramatic death, pass the shield to Bucky, who has a 9-picture deal and had great chemistry with Falcon in Civil War.
Iron Man - ride off into the sunset. Retire. Money whip him in 5 years for a cameo. End of a trailer, everyone loses their ****, that kind of thing.
Thor - maybe he succeeds Odin as king. He's getting a rebirth of sorts in Ragnarok so his popularity could be at an all-time high.
Black Widow - no way. There were two females in Civil War, out of 12 combatants. I'm actually quite certain she'll be getting her own movie in Phase 4. They want smaller, more personal stories, and Scarlet is still a star.
Hawkeye - maybe, but not much to be gained from it. He had a good brief appearance in Civil War so there's still fun potential there. Bring him out of retirement whenever. Also, Jeremy Renner has said they had some exciting ideas in mind for Hawkeye. Maybe that means dying, but maybe not.
Hulk - possible but he's such a fan favorite and he's great in small doses. A large chunk of the Ragnarok hype is coming from a little bit of Hulk. What is there to gained by killing him?
Scarlet Witch - again, barely any female heroes in this universe. Stories left to tell.
Vision - probably dead but hopefully not. After reading some comics he's become one of my favorites. Plus he gives Scarlet Witch someone to hang out with.
Falcon - keep him around for Bucky teamup at the very least
Nick Fury - sure. Why not? He's mostly outlived his usefulness and Samuel L's very big contract is up with these films.
War Machine - could die but he nearly died in Civil War so it might feel like a retread. He's not around that much anyway, sadly (for me)
Black Panther - not dying
Ant-Man and Wasp - not dying. They wouldn't establish the duo and then kill one of them 9 months later.
Doctor Strange - likely to get a sequel after Avengers 4 so not dying
Captain Marvel - not dying
Spider-Man - confirmed not dying
Guardians - probably nobody dies
My expectations: Cap dies in dramatic fashion, Bucky takes up the shield, Iron Man retires, Hulk lays low, Black Widow gets her own movie, Nick Fury dies, Hawkeye succeeds him as head of Shield, Vision gets rekt but maybe isn't dead for good.
2021 Avengers lineup can be pretty much anyone they feel like using, but you're not locked in with the same 6 like Age of Ultron.