I think we are *way* past the point of having to worry about spoilers. So I'm just going to say this.
I think that there's some possibility that Avengers 4 is going to provide a workaround to the expiring contracts problem for Robert Downey Jr., Chris Evans, and Chris Hemsworth, who play Iron Man, Captain America, and Thor. See, the problem is that you *can* do an MCU without those three heroes. But do you really *want* to? I don't think so. And we've already gone around the "Let's promote their sidekicks to wear the uniform" route in the comics with very little long-term success.
Except the fans are going to beat you up if you try to recast the parts. What to do, what to do?
It's pretty obvious that Avengers 4 is going to have *some* kind of temporal reset. Well, what if the temporal reset is imperfect and the result is that Downey, Evans, and Hemsworth are left outside the time stream when it gets rebuilt with new actors playing Iron Man, Cap, and Thor? Dr. Strange is, of course, aware of this and helps them shuffle off to heroic Valhalla of one kind or another.
And the recasting becomes part of the overall lore of the MCU. It might not stop fans from complaining, but it would be a neat bit of sleight of hand.
It's a thought...