I think a sort of Fantastic Worlds type concept (like the one developed by Universal in Florida), is the best way to go.
I could fit many areas and ideas and it has a lot of versatility.
Could combine elements of Beastly Kingdom:
1.A dark land, inspired by villains (think Chernabog the devil, or some sort of dark ride to include all Disney villains)
2.On the opposite side, a "good" land with the never built unicorn crystal grotto and maybe a Tinkerbell fairy ride.
It could also incorporate more Alien type lands, such as Avatar or..even more Star Wars.
Could also draw inspiration from Ancient civilizations, such as ancient Babylon, or Egypt or China, or Mexico (could include all sorts of rides and Pixar/Disney IPs or non-i.p. based rides).
It could include DisneySea Elements as well, such as having these lands themed as ports, or create a storyline along the lines of "sailing to otherworldly lands". Maybe it could have a Lighthouse as a center Icon, to further strengthen the Idea of Exploration.
With that being said...there is still A LOT to improve on the existing parks and sooo much expansion space left. I don't think a 3rd park will come in the nest 20 years, UNLESS they massively develop the other parks. The Main Park has expansion pads for every single land basically. It could double it's attraction line-up.