Final Fantasy XIII-2 is coming later this year and I have seen a trailer for it, which I will post here as well.
My interpretation of this trailer is this (Final Fantasy XIII knowledge required...):
Lightning has died and is now being brought back to life to do some sort of duty - which, according to an interview, is to find and revive those two characters that were sacrificed in the end of FFXIII.
So, it would make sense that those two characters are still alive many years later, as crystals, because they had completed their Focus given by the Fal'Cie.
Now it seems that Lightning will go out and search for them and hopefully revive them.
