everyone will come back. the infinity war duology is based on the infinity gauntlet (part 1) and infinity war (part 2) storylines in the comics, for anyone wanting a general idea of what will probably happen. everybody will be brought back ofc, but this would be a perfect opportunity if they want to recast any of the characters, since some of their contracts are about to run out, though passing on the mantle to a different character in-universe (ie, bucky or sam becoming captain america) is probably how they'll do it.