Seriously... Good luck if you try to put sephiroth in Eorzea's lore. I was surprised enough to see that Eorzea was one of the kingdoms of Ivalice, and thus got some shared lore with FF tactics and FFXII..

Lightning was included poorly.. She can travel between worlds. Well.. OK I guess ? Easy way to make advertisement without much creativity.

And now you want Sephiroth. Good luck with that. Sephiroth has NO chance to travel between worlds, keeps diing in his own world because everytime Jenova's relics raise him again, Cloud puts him down.

I know some people want him badly, but really, it would be a very poor idea. Sephiroth has nothing to do in the lore of FFXIV..