Hagoromo as a final villain would be odd, as he'd need a way to resurrect himself, which could be what the hokage are for. He doesn't seem like the villainous type, so it would be doubtful
However if it is then there is only way Kishi can show that would be tricking Edo Hokage to revive him somehow or fill with chakra so he ressurect himself then go to Kaguya dimenssion assisting Naruto and Sasuke with finishing his mother then steal their powers somehow and overtake Kaguya.
It would be too complex for Kishi to do that, not even mentioning how stupid and contradicting to his previous actions.