"For Lightning, her story ends here in Lightning Returns: Final Fantasy XIII, as this game is meant as a finale to this series. But we know that Lightning is an extremely popular character, even when compared to other Final Fantasy characters," Toriyama said. "So there is a possibility that Lightning would appear in other future Final Fantasy titles, not as a protagonist, but as a guest character."
Lightning wouldn't be alone if this were to happen - heroes from past Final Fantasy titles have appeared in games like Final Fantasy Tactics, Theatrhythm Final Fantasy, Dissidia Final Fantasy and the Kingdom Hearts series. At least keeping her role limited to "guest character" will (hopefully) save us all from "Lightning Returns 2: Electric Boogaloo."