I honestly see making a fifth gate a great idea...once all four other parks are up to par. I think 5 is a good number. I don't really think I'd build more than 5 though...so if they ever do plan on a fifth gate, it had better be something pretty amazing!
To be completely honest, I'd like to see another E-ticket added to the MK, rides that are rundown fixed up(Peter Pan, Big Thunder, Splash), and something to replace Stitch.
In Epcot, get something in the place of the WOL pavillion. figure out something with the god-awful Imagination pavillion, and a Universe of Energy redo. The countries could use additions as well.
Hollywood Studios...what a thematic mess. Expand Pixar place. Fix up/update Great Movie Ride. Adios Backlot Tour. Add Indy ride. Adios Sounds Dangerous. Adios Animation Tour...open that area up with a new walkway leading into Sunset Blvd to help with park traffic flow.
Animal Kingdom...getting Avland...great. How about adding more rides elsewhere in the park? Fix Everest already.
Waterpark additions...both Typhoon and Blizzard need something new.
After all that's said and done...bring on Gate number 5!