Honestly, if someone hasn't played since R/B/Y, they'll do some research, be completely appalled by the new Pokemon, and not buy X and Y.
The only thing Nintendo could do to keep those of us who've been with them since '97 is to deepen the storyline. There's no going back now that the Gen 5 Pokemon are actual Pokemon, and the mechanics are set in stone. The Pokemon and the mechanics will not be the draw, it'll be the storyline.
However, a deepened, darkened storyline would mess with their E rating which is something they'd probably not wanna give up in a main series game.
Either way, I predict that the storyline will be the biggest factor in the target demographic.