I agree wholeheartedly with Lightning.
Pokémon GS was originally called Pokémon 2, if I'm not mistaken. It implied a different story, and in GS's case, it had new Pokémon, a new region, new characters, new mechanics-- new everything; however, GS followed a few things from the previous game such as having Team Rocket as the main villainous team, and having a lot of characters from the previous game reappear. I'm hoping for BW2's case, the "2" means a new region, new protagonists, or at least a completely new plot. They can retain the Generation, it's alright. As long as they make it like how other video game developers handle their sequels, then I'll be happy.
Like Cello's notion, having a "2" in the title yet giving something like Yellow/Crystal/Emerald/Platinum would be false advertisement. And it would make me feel stupid.