If [Insert New Region Title Here] is close to one of the other regions, than it's possible, in fact probable! I would love to see Hoenn in a 3D environment, but sadly, I don't see that happening since it's on an island sort of far away from the other regions. I kind of sad we couldn't visit Kanto in Emerald since you could go to 2 of the Sevii Islands (mind you, that was with an event exclusive item...). Those two islands were also visitable in FR and LG, so why couldn't we visit the rest of them in Emerald, and then go to Kanto from there. I mean I'm not complaining since Emerald already has so much to offer, but it would be cool...
In the case of X&Y: If it's close to Hoenn, then that opens up tons of possibilities! Hoenn could provide access to the Sevii Islands, which could provide access to Kanto, which would provide very easy access to Johto as well! OOH, I just got REALLY excited! Unfortunately, though, that seems very unlikely... but that would be awesome!!