Instead of a 5th gate I would take:
1) A new land for AK
I know AK just got Pandora, but the 20 year old park needs more attractions.
2) Expansion of Adventureland/Frontierland @ MK
An Adventureland expansion would balance the addition of the new Tron E-ticket and add space to a land that has been cramped by the addition of the Aladdin spinner.
A Frontierland addition that removes the dead end @BTRR would help with traffic flow and add capacity.
3) Add new counties to the World Showcase in Epcot
It has been over 25 years since we received a new pavilion. Nuff said.
4) Develop an Indy mini-land at DHS.
SW:GE will be so popular, DHS will need more attraction to handle the crowds.
5) Replace the shows at DHS.
These show have worn out their welcome. New show = new interest and shows are people eaters.
This are my top 5. However, I would also like a retracking of Space Mtn., a redo of Imagination, expansion of Kali, Yeti fix, CoP new scene, MK Theater, redo Fantasyland facades, new Epcot entrance (remove tombstones), Fantasmic refurb or new show, and a SW expansion (I know it hasn’t opened yet, but it will need it).
All of these things should be done (and more) before a 5th park is even considered.