Seriously doubt there will be a fifth gate ever. But for the sake of argument, if they were to develop a fifth gate I can't see the entire park being villain themed.
True BUT .....
You know how there is the line of Disney books that tell the other side, like Wicked style. They could have attractions based on the stories, including the "precious" characters but looking at it from the Villains point of view. Not just the traditional, could have Darth Vader, Zurg, etc.
I know it will never happen, but they really need to stop ignoring the cash cow Villains are. Two Unleash the Villains completely insane and packed, Club Villain was great but they didn't diversify to get repeats AND you had to have a park ticket too, Villains After Hours did well, Villain lines at
MNSSHP always long ... for many of us they are our people.

I will throw money at them for Villains merchandise and events ... a park - would be the cherry on top.
But I'll settle for events, character meals, more merch AND .... a Haunted Mansion hotel AND a Haunted Mansion TS in MK. [move HOP to EPCOT where it belongs and use that valuable real estate for HM TS with varying rooms like BOG. Ballroom, Cemetery and Attic.]