My ending would be Jack killing Future Aku then everything is all sunshines until Ashi knocks him into a wall and starts giggling.
Jack goes "Ashi what are you doing"
Ashi goes "Whos Ashi?, yoo hoo samurai I never left the building"
Jack then goes "Aku is that you how I destroyed you?"
Ashi goes "I am not Aku Aku created me however I am that entity that was just trying to kill you moments ago"
Jack then goes "But Ashi broke free!"
Ashi goes "No no I made you think she freed herself Ashi is gone Jack Im now in charge of this body and my first idea was to get rid of Aku'
Jack then goes "Why?"
Ashi goes into her Aku suit and starts laughing" So I can destroy this world and everything in it"
Jack then goes "Ashi please fight it get out of it"
Ashi goes "I guess I am just like my mom after all no hard feelings my love but Ill have to dispose this sword so nothing can stop me"
Ashi then throws the sword into space.
"And its gone, oh dont be sad my love now we can be together forever while you watch me destroy this very world you despise so much its gonna be great Jack :)"
Jack then falls to his feet so depressed that he cant even think anymore.
Ashi walks over to him "He'll get over it my love for him will work this out".