Submission: A submission because the ask box has a character limit.
(This is an idea that’s angsty af an probably would be better going to Holly but Imma share the tears anyway.) Tobirama has always been super smart, moreso than Hashirama when academics are involved, but as the clan wars drag on he starts going a little crazy. The first time he uses a human for Edo Tensei, it’s when he uses some faceless, nameless ninja from the Land of Lightning to bring back Itama (who is a little confused at first, but is more than happy to see his brothers again, and then more than a little hurt when the look on Hashirama’s face is one of fear directed at Tobirama). The Senju are horrified, such a jutsu is downright sacrelige. The Uchiha are intrigued, they’ve lost so many over the years, and they invite Tobirama and Itama to join them, promising to help Tobirama in his research so he can find a way to truly bring their loved ones back to life. He discovers the truth about the Rinnegan, and sage mode, and how the chakra of the bijuu can bolster that of a mortal. He seeks out Matatabi, begs her aid and allows her to basically possess him of both their own free wills. Just… sorta crazy Rinnegan-wielding sage jinchuuriki Tobirama who was basically adopted by the Uchiha.