Gilgamesh is a unique case though, as his character seems to literally be traveling through dimensions and is the same character in almost all of his appearances in a Final Fantasy game. So he can appear in more than one game and makes sense lore wise. There's a very good chance more classic Final Fantasy bosses will appear (because, they already have!), just as new incarnations and not literally the same boss from previous installments. So I don't think Sephiroth would work because in my opinion it would be silly to make a new incarnation of him and it would be just as bad to break lore and bring the actual Sephiroth over from FFVII.