I believe future worlds will be either; Polaris or Mirage. Polaris is mentioned in Pirates101, but also much so in Wizards before it. I would love to see Mirage, we can see where the fantasy palace, flying carpet, and genie originates from. I see it as an ancient persian dynasty type world. I know some of the people are cats, I think the villains may be snake people.
Polaris has hinted at penguins, which sounds interesting. Obviously a snow world.
I would like to see a world based off of the sky. Have parts of the world be broken apart and bird people reside there. Warriors could be eagles and the common folk be other birds of all sorts. The villains could be vultures.
Also, another is an underwater world. Celestia was more an underwater world with sharks, fish, and crab people with water moles on land and the occasional alien presence. I would like to see more of an Atlantis type world, friend brought this up.
I love the whole forest area, but I think its over played, sure a native american themes world, or wild west rather would be cool. The native americans can be buffalo people.
Finally, I would love to go to another modern world like Marley Bone. As for what the theme would be, I do not know.
What about... space? :D We could be in a space station fighting enemies that have secretly gotten aboard... What do you think? Then, in the space station, you could take space ships to different planets. :) Please respond!