MC gets reincarnated as Obito, after the death of Madara. He stays evil till the end.
Very simple. Some suggestions though.
My only idea would be adding research of the Hashirama Cells by using a Byakugan. With this he could get more stable Cells and bear the Rinnegan easier. Another idea would be finding some Madara Cells and merging them too, which would grant a personal Rinnegan.