What if Jack and Ashi went back to the past, but instead of just disappearing all at once because Aku doesn't exist, Ashi's body is deteriorating because she's unstuck in time like Jack was and instead of becoming immortal, she's just crumbling away. So instead of dying, Ashi has to go back to the future and the series ends on Jack in his timeline and Ashi in hers, trying to rebuild the world after Aku's demise. It's bitter because they can't be together, but sweet because they still have the memories of each other and the series ends with a shot of them under the same tree, apart, but ultimately happy they met each other.

Would that have satisfied everybody?