Really, for them to add Hoenn or any other region would be ridiculous. Completely. It would actually make LESS sense for them to be there. I mean, the ONLY connection they have to every other region is that they have a Pokemon League, Pokemon Centers and Marts, Trainers, Pokemon. Not only that but it's be said many times that the Unova region is more than likely on a completely different continent. Why and how on Earth would Hoenn or the others be connected to it? As much as I believe there will be an R/S remake, I don't see it happening too swiftly, nor do I see Hoenn in this game. Hoenn had its own game, and so did Unova, and that's how they should stay: in.there.own.game. Putting another region in there just because "it makes sense" would be ridiculous. Pokemon have QUITE an amazing ability to travel. It only makes sense that they would be able to make it to Unova. All the way from natural migration to people who travel around the world bringing Pokemon to the Unova region.
So, no, I do not think there will be any other region in this game, and I'm quite pleased at that.