They're contractually barred from building any Avengers attractions in Florida, even though they own Marvel. (Poor them!)
Their best bet would be to do a big Star Wars expansion in Hollywood Studios to coincide with the new films. At the very least, it would be a bigger draw than the already in-progress Avatarland at Animal Kingdom
I can't really imagine them adding another park. I'd be cool with them making a Magic Kingdom 2.0 type park with a proper Pixar area (not just Hollywood Studios' 1 Toy Story ride) and whatever else caught their fancy. Doubt it'd happen though. [MK2.0 because the existing MK doesn't have room around it to expand further and I hate the thought of them getting rid of nearly any of the existing rides, although I feel like Tomorrowland needs major updating].