I think the issue is they've been incredibly encompassing and broad with the themes of all of the parks. There isn't a lot left that isn't cutting into the other parks in some fashion, especially since they decided to double down on IP, but only IP they fully own. Something like a "music" or "video game" theme wouldn't work given the amount of licensing they'd need to do. A Marvel or Star Wars park defeats the purpose of pushing GotG into Epcot and making Galaxy's Edge in Hollywood Studios.
The one thing they don't have is a real "horror" presence- ToT and HM, sure, but the surrounds of them aren't particularly spooky, it's more "happens to be in Hollywood and haunted" and "happens to be in former-colonial USA and haunted". But I'm not sure how viable a real horror park would be given the presence of families with kids under 10. Epcot and Animal Kingdom as concepts were at least just "too boring" (not my words) for kids and families could just drag them along anyway. A Horror Kingdom or something like that would potentially give the kiddos nightmares, and some aspects of Disney already do that without meaning to be scary.
I guess they could do a fantasy-themed park that isn't based on IP given Fantasyland is "the Disney animation land" first and foremost, but a lot of what they could put in there could be put into AK or World Showcase too.
Then again this was an issue with Animal Kingdom too- the thought was "well, there's the Jungle Cruise, do we need an animal theme park?" yet, that panned out. But I'm just trying to think from the viewpoint of current Disney management. If it's not completely uncovered by the other four parks, and isn't something they can plug IP Disney owns into, why do it?