Universal never used the Defenders so I believe that family of characters is open. However Universal used the Hulk, Silver Surfer, and Namor the Submariner, so in essence they used the Defenders, if not in name. Dr. Strange has appeared in the parks so we know he is open. Dr. Strange was also an Avenger years after the contract was signed. So we do know there are limits to the characters inclusion in so called families use in the parks.
I would imagine this means that characters that became Avengers, X-Men or Fantastic Four members after 1994 are possibly usable in the parks. We also know that Universal has used less than 1% of Marvels characters. Maybe a total of 30 to 40 of them.
I don't believe the contract mentions the places of the Marvel Universe either, so could Disney build Wakanda? I think they can. Can they use Black Panther family characters? I would have to say yes, if the characters were created after 1994, or not a major character in the "in use families".
Would Disney build Wakanda without the Black Panther? I seriously doubt it. Would Disney build the 100 acre woods without Pooh? I really doubt it.
Now if Disney wanted they could use a character like Kazar and build the Savage Land unless he is included in one of the "in use families". I suspect he was not on the list and if he was, he shrank off of the list due to non use.
Marvel was in dire straights at the time of the contract however I do believe they knew the value of their IP and that the list of usable characters was not unending and MCA at the time probably didn't see value in more than 30 to 40 characters anyway. So I suspect the list is shorter than most would think and also longer than most Marvel fanboys would like.
I would bet there are characters connected to Thor like the Warriors three or Odin that are not on the list and that each of the Avengers has a list of supporting characters not in the Avengers that are usable.
In all honesty I was extremely disappointed in Marvels Superhero Island. I thought it was very cheaply done and a huge cheat. Two of the rides can be found almost anywhere and the theming is a joke. Those rides really don't meet the standards of local theme park attractions.
The Hulk is one step ahead and Spiderman is now very dated. Painted plywood signs make the area look like it was never meant to be more than a placeholder. By building better and better more themed attractions Disney and Universal have almost caused this land to be a breach of contract. If you have been there can you honestly say it meets the standards of Potter, Suess, Pandora, TSL, or Galaxies edge.
Disney should demand that they improve the areas and attractions or loose the rights.