8 Things Marvel Could Do After Phase 3
6. The Inevitable Recast
Marvel has previously replaced Terrence Howard with Don Cheadle in the Iron Man franchise with nowt more than a wink and a nod (Cheadle's first line in Iron Man 2 is "It's me, I'm here, deal with it."). Given that there's precedent for replacing supporting characters, it wouldn't be a stretch for Marvel to recast their main players once their contracts are up.
Downey in particular is almost over the hill in regards to action movie theatrics, so if recasting is an option, he'll be the first candidate. The trick is finding someone roughly the same age, or only slightly younger - Jon Hamm springs to mind. As jarring as a recast would be, it would probably be the safe bet from a business standpoint.
A certain amount of trust comes into this option by encouraging viewers to accept the replacement. There is a certain amount of ridiculous comic book history that actually encourages this kind of replacement; I'm sure many of you will remember when Tony Stark was de-aged and turned back into a teenager for some inane reason, even as the rest of the Marvel heroes remained the same age. Nat Wolff could be an option here - or any of the other failed Peter Parker candidates, for that matter.