A 5th gate won't happen any time soon, if ever. Each park would need at least 15-20 attractions each before each park reaches it's ride capacity. Not including shows.
DHS needs 3-4 more projects (Monsters, Animation Courtyard, Echo Lake + 1 More?) before it's a "full park".
EPCOT needs 2 more countries with rides, a few added rides to existing countries (Like Remy) and a few pavilion revamps (Mission Space, Imagination, Wonders of Life)
MK has room for at least 1 or 2 more expansions in adventureland and beyond and actually using some of the closed building.
AK has room for at least 3 new areas (Like a Lion King area and somewhere between the park and conservation station) and more revamps (like Kali) and expansions in Pandora. Also a night time show.
I don't believe a 5th gate gets people to stay longer and spend more money. If most people go for 5-7 days with 3-5 park tickets does that get people to buy more hoppers? Is having people go to 5 parks instead of four parks with an extra MK day make more money for the company?
I just don't know what a 5th gate gets TWDC.