KAAAAAAAANG.
I don't expect the "classic" roster to stick around after Infinity War, so we need them to face off against one of the most classic villains before it's the New Avengers or whatever they decide to do. RDJ and Chris Evans are gonna leave based off comments they and others have said. But Iron Man and Captain America must meet Kang.
RDJ has said that he is in negotiations to appear in more Marvel films. Just not Iron Man 4 for the foreseeable future. Evans is only 33. He wants to do other things but hasn't ruled out coming back as Cap.
They will both be in Avengers Infinity. Even if Cap is killed in Civil war I could see them bringing him back via the time gem.
I'd have the 1st part of Infinity as Thanos collecting the gems. He destorys the Nova Corps. Kills the Collector and takes the Aether. Collects the Tesseract and Scepter from Loki. Gets the Soul gem from Warlock. Guardians then flee with the time gem to Earth. Here they get Tony's Avengers to help them try and defend it from Thanos. Thanos easily defeats them.
Tony then uses the time gem to go back and save Caps life and tells him what will happen in the future. Tony returns. Thanos tells him he was a fool that the human body isn't strong enough to use the power of the gem. Tony tells him he was storng enough to do what he needed. Then all the rest of Earths heroes and Asgardians etc... turn up led by Cap.
Thanos tells them they are too late that he has what he wanted and teleports away.
Cap goes to Tony. Tony (dying) tells Cap he's sorry that he should have listened to him. Tony then dies.
Cap tells Tony's dead body that he will be avenged.
End of part 1.
Part 2 will be Marvel heroes taking the fight to Thanos.
They will defeat him although many will die including whole galaxies. Warlock then gets the Infinity Gauntlet. He puts everything back to how it was before (except Cap is now alive) for everyone involved it seems like a dream.
Can even have the ending with Warlock facing the Living Tribunal and other great forces. Where they tell him that he has to give up the power and split it.