If a 5th gate happens it would include Marvel stuff in decades after Disney is allowed to use it in parks, maybe a heros and villans type of park. Could also include all their non marvel lineup of course. I dont see a need to make a 5th gate yet. Still tons of room to expand AK, could move EPCOT into parking and the ball would be in the middle and not the front, and HS getting expanded now plus could do more later as well. MK could eat up some backstage areas and expand, and Tomorrowland could be nearly totally overhauled.
Universal isn't likely to give up the rights until Marvel has dropped considerably in popularity and if that happens it wouldn't make sense to build a third gate themed around it.