Marvel has been doing such a fantastic job with their movies and the way they interconnect. It's a damn shame that Spiderman and the X-Men are cut off. Don't so much care for FF, but that's just me
I'm sure they will do what both Marvel and DC has done dozens of times in the comics. That is make Captain Ersatz of spider man/x-men characters and include them in the movies. Then they do that until they can buy the rights back. Hell as shown in this link Shazam is literally a Captain Ersatz of the original Shazam. I imagine Marvel will likely do something similar in future movies. That is if Fox and Sony don't work with them. If they don't work with marvel it won't be hard for them to make a Captain Ersatz. The great thing about the fact that both Marvel and DC opened up comics to the multiverse, is not only do they have the possibility for infinite stories and dimensions, but if they want to they can sell off the rights to one dimensions version and make a movie with another.
Take spider-man for example. Sure they can't make a "Spider-man" movie without permission from Sony Pictures, but that doesn't stop them from having a version of him in one of their movies. Take the planet Doom time line for example. Doom goes back in time and mucks with all the heros so no one can stop him. Peter Parker never becomes spider-man but instead dawns a cool black outfit and calls himself "Slinger" Even changes his phrase "my spider sense is tingling" to poke fun/give tribute to the populer DeadPool meme by changing it to "My common sense is tingling". So there is no reason they can't do something like that with all the characters and they have more than enough source material to run with.