The biggest question in this area is not what would cause Universal to completely relinquish the Marvel rights, but what would be necessary to give up the exclusive rights to Marvel east of the Mississippi.
As it stands, Universal is sort of in stasis with the Marvel section. They really can't add anything new (as Disney may be loathe to approve), but have to maintain what exists to keep their rights. And their attractions are based on four main categories: Hulk, X-men, Fantastic Four, and Spider-man. Two of those segments should have major films coming in the next several years. Could Disney offer access to the film versions of those characters in exchange for the right to build Iron Man, Captain America, Thor, Captain Marvel, Black Panther, etc attractions in addition to what is at Universal?
That's a more interesting question that could reap benefits for both parties.