I'm not sure I'd honestly like him to come back just having turned out to survive or something. Him killing himself at the end of Stormblood was a pretty logical end to his character arc. He spent his entire life seeking a greater challenge, specifically one that could finally equal him in terms of strength and their distance from humanity. When he finally achieved that goal, even finding a warrior who could defeat him, he could finally be happy. He basically had been living his entire life for that singular moment, and upon achieving it there really wasn't much else for him to live for. I'd honestly prefer him to be a clone or (as my pet theory goes) a corpse puppet for Elidibeth. That would make much more sense with his character progression.