Still one question... Why?
I think many good themes are being suggested, but at some point someone has to introduce some practical questions.
Why should this be a separate park? Who does this appeal to? What groups need to be included that are not interested in this?
As I said about my original suggestion, there are reasons for a new park like the one I suggested. What would make my park different is that it would be aimed at including more coasters in the park to compete more directing with what Universal is doing. Along with the "thrills" of coaster, other "thrills" could be included. I have explained that people do want more exciting and thrilling rides than when Walt Disney originally designed his park. I also explained that this park might provide a place for Disney villains who right now do not have much place.
The suggestion of myths and and legends has some interesting potential, but why? That sounds like a great "land" somewhere, but not a whole park. Right now I cannot say Disney "needs" a whole new park for myths and legends. Who does it appeal to? This idea is very attractive to males, but not very fun for females. It also does not sound very attractive to children. What is included that would be attractive to little girls? What would you include that would be more Kid-centered?
It is also not clear what kind of rides or attractions would be there. Why would this be fun? What is fun about a land with Greek statues and temples?
You know, many people prefer Disney as most of rides aren't thrills, just enjoyable things for everybody(mostly), I for one don't enjoy massive thrills, which are more present at Universal, so medium roller coasters like Space Mountain and fast paced thrill rides like Test Track and Dinosaur are what I enjoy. A park that only targets younger children and thrill seekers(which is essentially what Islands of Adventure is) isn't targeting all sectors, which is what Disney needs to do, there's no wonder why Disney parks are much more popular than that of Universal.
A new park would of course include thrills, but they wouldn't be cheap looking rides with heaps of inversion, like the Hulk, I think the limit of intensity for Disney is with TOT and RNRC. Also, Hollywood Studios is becoming the park for thrills, and it's doing that well, as it has stuff for everybody and is soon to have more simulator thrills in Star Wars Land and the roller coaster in Toy Story Land. So, truthfully, if Disney were to create a new park, which there isn't really any need for anyway, it wouldn't be a thrill based park.
Epcot is about the real world, the future, and innovation, Magic Kingdom is indeed themed after Disneyland(there's no specific theme fir the whole park), Animal Kingdom is about nature that does, did, and could exist(real life or lifelike nature, not just anything with animals in it), as well as conservation, and Hollywood Studios is about films. A fifth park would have to have an inventive theme that is different to the others, even with a different theme park layout, I don't have any good ideas for a new park's theme, but I see no need for a new theme park anyway.
I'd build a new water park, as big as Volcano Bay(but better themed of course) with some amazing centrepiece. This would add another day or good half of a day to every holiday, you could even include a river rapids ride and a log flume, themed of course, as these would be tremendously popular as nobody cares about getting wet, unlike in a normal theme park, these would mean that it becomes more popular than Volcano Bay.