Mattias

This is perfect have him in a coma like state where he can acsess his past lives and treason with hashirama in his mind. It gives the franchise to explain exactly how reincarnation works. Then he comes back stronger than ever, becoming the most powerful, to powerful. Leaves to the moon or becomes a nomad like the sage of six paths. The next gen can continue to grow and evolve but he is always watching over them in case of any really drastic case. In his comatose state he learns more about his brother’s past life and before becoming a nomad they can write an arc ending the conflict between the brothers through sasque and Naruto meaning this will be the brothers last reincarnation as the shinobi world evolves and finialy having the frist utchiha hokage be sarada.