Not sure how this will end up being a "happy" conclusion. I personally thought FFXIII ended on a bittersweet note. Then FFXIII-2 showed up and basically blasted that out of the water.
NOW, Lightning awakes hundreds of years in the future, Cocoon has turned into a moon-type planet in the sky and everyone she knows is dead. Lovely.
Yes, Fang and Vanille will most likely return in some form (hopefully) but damn everyone else. Toast. --- Currently playing: Guild Wars 2, Fallout 3, and Diablo 3 (There I removed that heathen of a FF game.)
Lightning awakens in Novus Partus, a world brought on by the dreams of those in crystal stasis. On the 13th day, she defeats the source of Chaos, which saves what's left of the Mortal Realm, but is still stuck in crystal stasis. Over the years humanity slowly dies out until Noel is born again. Lightning awakens from crystal stasis, to see the end of the world, and is now powerless. Noel and Lightning live out the rest of their lives, and start a family. Hundreds of thousands of years later, humans finds Lightning's remains...she is the ancestral mother of all humanity on Earth. Her and Noel, were Adam and Eve!!!!!