It is possible for them to make DLC regions or islands. They surprised us and changed a lot of what we are use to in Unova. Nothing is impossible, but I would love to go back to Hoenn. I don't see us getting remakes any time soon for Hoenn, so maybe downloadable content to or part of Hoenn.
Also, people need to keep in mind that the routes reset in Kalos.
And to the person who said something about them still having a lot of land in Japan, a good amount was used in Pokemon Colosseum and XD: Gale of Darkness. I aslo think some was used in the pokemon ranger games. (Sorry for getting off topic here.)