Can’t do much outside of revising the land a la Hulk. Uni’s contract with Marvel specifically entitles them to these characters, but says they’re to be the comic versions. No MCU, no Earth’s Mightiest Heroes (sorry).
As for character absences, the status of their appearance (or lack thereof) is reflective of their popularity when the land was being designed/built. No one thought much of Black Panther prior to 2016.