I love thinking about a 5th park. I don't think it'll happen anytime soon which is unfortunate considering the new resorts being built. I personally dislike how Hollywood Studios is a bit of a hodge podge of ideas.
I feel like there would have been a decent case for a Pixar themed park like a Pixar version of Magic Kingdom where it captures all the imagination, rides and character meet and greets over the many movies. They have Cars Land in Disney Land they could have replicated to create a fair portion of the park, and I'd move the Lightning McQueen Racing Academy show there. Of course the Ratatouille ride could exist at Epcot as planned just like a Mickey and Minnie ride can exist in Hollywood Studios and not in Magic Kingdom. I feel they could even go forward with this idea if they wanted to, even with Toy Story land in Hollywood Studios as Pixar still has other really popular franchises. There's also nothing stopping them from having another Toy Story attraction here though I probably wouldn't theme a large section after it now that Toy Story Land exists, unless you do something movie specific like a Woody's Roundup retro theme.
I think there was a real opportunity to capitalize on the Star Wars franchise as it's own park too. The issue with my aforementioned ideas is they are all a bit too specific and themed, perhaps almost niche.
I think as
@Davey Jones II said, Disney Sea (I've never been, but have heard good things!) would be a really cool option and there's a template to follow there.
Unfortunately Universal Orlando contractually has rights to Avengers on the East Coast so something that would incorporate The Avengers is off the table. I think something Marvel related would have made a lot of sense as again it's specific but definitely wildly popular right now and even if it slows down, there's generations of fans (old school comic book lovers and those raised on the MCU) that could make it marketable. That said, I wonder if concepts and tech they're using for Avengers Campus at Disney Land could be re-fitted for another Disney owned property to save money on R&D if they wanted to come out with a new park that would use some of those concepts from that particular area in Disney Land.
Again, I think they took some great ideas and stuck them in Hollywood Studios when you probably could have gotten another theme park out of these.