What Disney should do is simply pay Universal the ransom money to allow it to use whatever Universal is not currently using. They probably could never get Spiderman or the Hulk. But the use of some characters on a poster? Disney should pay to get them back. Seems silly to have Iron Man and Thor meet-and-greets in every Disney theme park except WDW.
I'm pretty sure disney and universal both don't want to do this. Universal likes having the rights to the characters and disney makes money from them having the characters. In order to get them back is would cost 100s of millions if not billions to do so. Then universal would have a bunch of rides without a theme, spiderman would have to be demolished or completely redone and I'm sure they would expect Disney to pay for this that's why it would cost billions. Much better for both sides to stay the way it is.