I think we'll see a 5th theme park, but I refuse to speculate as to what it will be. Maybe a new non-DVC resort or two, but I'm sure we'll see them expand DVC more. Hopefully they'll either finish Pop Century or clean up the unfinished side.
As for the parks themselves, I doubt most of the recent film-franchise themed rides will last twenty years. Stitch, Monsters Inc, and Nemo will probably be rethemed to the franchises of the future, or (hopefully) revert to being original material.
There's obviously an update in Mission: Space's future, since in 20 years, it will only be set 6 years in the future. There will probably be a new Soarin' film, and hopefully they'll add a new theater or two to ease up on the lines.
Speaking of Epcot, hopefully we'll see a new country (or two) in World Showcase, and Wonders of Life will likely be replaced. Universe of Energy will probably not last long in its current form. (how many kids today know who Bill Nye is? sad as that is
![Frown :( :(](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
)
Otherwise, most Future World pavilions will probably be updated in some way.
This was mainly about Epcot because its my favorite park, but I'm sure other parks will change just as much. Just think of how WDW was in 1990 vs now.