If the unite regions in any later games, the only thing I'd be comfortable with guessing at is a Hoenn/Sinnoh joining. But I'm not even certain they'd join regions in any games again, anyway.
All of them in one game? That'd be overload: Pokethlon, 4 Leagues, 2 Frontiers, 3 Safari Zones, 2 Contest Halls, Battle Tents, every Crimnal Org., the list would be endless...
I'd like a more story-line simplified release: no criminals to fight, no rampaging legendaries, just you, the league/conference of the region, gyms and battle tents.
And a place in the wild where you could actually get good xp yeild to match your level.
Frankly, if they want to be really economical, they would merge the main games and remakes into one. This generation would feature a new region, and Hoenn as the secondary region. Generation VI would then have its new region and Sinnoh as the secondary region. However, this is ignoring the economics that making remakes makes Game Freak more money than not making remakes.
The idea is wonderful from a players point of view: Being able to go to every region around. From a business point of view, this would kill the possibility for remakes, which are economically lucratrive, or else Game Freak wouldn't keep making them. That, and oversatisfying your fans is very rarely a good thing.
If this game has two regions, I doubt we'll be familiar with either one. Most definitely, it will feature one new region.