I have an other idea. At some point, Ashi manage to get the upper hand, just when Aku is about to destroy the sword. They are in a mental struggle and Ashi plead Jack to destroy Aku, even though it will kill her too.
Jack don't want. Aku laugh as he is about to regain control. He say even if he defeat him, he will be damned to solitude for the rest of eternity, that he will never get back in time as all time portals are destroyed and that in the end, Jack don't really want to destroy him because he know that in the end, they'll only ever have each other that he could even consider ensuring the immortality of Ashi if he considered working for him. To which Ashi answer that Jack won't have to choose her, that all time posrtal might be destroyed, but there is still one thing. then Ashi channel Aku's power and generate a time portal herself, Sending Jack back in time.
Re-emerging back in the past, Jack immediately understand that he has to save his people and honor Ashi's sacrifice and finally defeat past Aku, sealing the future he knew forever.
Years later. Jack has grown old. He has become emperor and then resigned from the throne for his nephew. Wandering as a lone samurai again, he come back to a familiar place.
-I have been waiting for you, samurai.
-Curious, I thought that for you, it would be the first time we would met
-Time is tricky like that, Samurai, and I got special privilege.
-This isn't any kind of portal, isn't it? It does not just sent you back on forth the river of time.
-Smart guy. Yes, it is a gate to any possibility, to what could be and even to what was once possible yet isn't any more. but there is still one thing to figure out, Samurai...
-Do you deserve it?
draw the sword
-Let's find out, Guardian!