Problem is Marvel's most popular characters are in the "families" of Spider-Man, Fantastic Four, X-Men, and Avengers. If you're a Marvel Superhero, you almost certainly served time in one of those groups, it's like the jury duty of the comics. You could make the argument then, that almost every Marvel Superhero is part of one or more of those groups.
Problem with that argument, which I'm sure Universal would love to make, is that the contract very clearly presumes there are Marvel characters who aren't part of one of those groups. If Universal claims that Doctor Strange and GotG are part of one of those families (and they were for a short time), then Disney can say, OK, then, who isn't part of those families... give us a list of all the characters we can use in Orlando. And Universal would be hard-pressed to find any that would meet that criteria of having absolutely no relation to those four 'families.'
So, it's going to be difficult to find the few superheroes that WDW can use. Just look how obscure GotG was to the general public. They're going to have to elevated a lot more little-known characters and puff them up to make them park-worthy. That'll take years and years.