B/W 2 were enticer versions, since they didn't have plans for a third one like usual. They were meant to give fans something a bit new to play through so as to catch any late-comers to B/W and keep B/W popular long enough for X/Y to be released. Many are hoping there is a Z planned, it's possible, we won't know until someone completes the game because every trio game has three main legendary pokemon in the story. B/W only had two, and not even a rumor about a third, that's why so many of us knew there would not be a Grey.
I'm kind of hoping for a Z a year from now, not to play, but to invigorate the fans and keep X/Y interesting even longer. The trio games tend to boost fan activity a lot.