Improve the parks they have should come long before adding a fifth gate.
AK is getting a lot of attention with nighttime entertainment, the enhancement of Harambe and Pandora coming.
DHS is in great need for at least three more attractions and a couple restaurants. While I would prefer a SW land with two or three more SW attractions I believe the general public would appreciate another SW attraction or two with a little something else too. Change out VotLM, maybe BatB shows with some more current IP such as Frozen.
EC need so much more in Future World, replace EO, upgrade Figment (or preferred to revert to original), add capacity to Soarin', replace Ellen (PLEASE!!!) with something relevant and add a couple new attractions to round out the park. I would love to see anther country or two in the World Showcase but cannot eat at them all as it is (and that is with two week vacations) so I would expect it would just compound traffic congestion that already happens in some areas.
And, the water parks need a little attention too. They should each have a new attraction added at least once every five years. Add in a swirling bowl in Blizzard Beach and maybe a surfing area in Typhoon Lagoon or something, just add something to make people go there.
Disney Quest needs some updating in a BAD way, the 80's are long over. They could completely reskin the facade to create a giant Flynn's arcade in the front where the gift shop is with just a few games then use elevators to access themed floors with TRON, Game Central Station etc. for areas. I think it would greatly enhance the experience overall.