It has been announced that Lightning will in deed become playable for Final Fantasy XIII-2. Though, its not without a price. We already knew that she would be available as a coliseum boss, which would presumably allow you to add her to your party upon defeat.
Lightning will also have her own secret episode DLC, and other Final Fantasy bosses will make their appearance. Square Enix also plans to expand the story outside of the game with novels and such, including Sazh, Dajh, and Chocolina.