We all know that Feige can't make any formal plans for the returning heroes now, but we also know that he's been thinking up informal contingencies for them since about 2006, which is when he was appointed co-president of Marvel Studios. Every time reporters asked Feige whether he thought that Marvel Studios would ever get all of its characters back, his replies would be cagey and noncommittal but it was clear that he desperately wanted it to happen. And, I believe, it was equally clear that he had some very good ideas about what he would do with his prodigal heroes if and when they returned.
Now that the Fox acquisition is all but a done deal, Feige and his brain trust are doubtless spending part of their time dreaming up scenarios for the FF and the X-teams. Whatever we fans can think up, Marvel will create something better. Whoever we fan-cast, Marvel will come up with actors who suit the characters even more perfectly. All they have to do is wait until the word "Go!" comes down from on high.