I could see that. I have a different idea that might strike more of that bittersweet tone.
>Jack makes it back to the past at great cost, Ashi, Scotsman, others die
>Jack returns to immediately after Aku flung him forward and strikes Aku down
>However, he cannot kill Aku outright
>He's still unaging from the time travel
>Jack resolves to stand guard over Aku for the rest of time so that he may never rise again; it's a lonely existence, but Jack finally resolves his guilt and is at peace
>Final shot is Ashi from the new, brighter future finding herself drawn to the area where Guardian Jack is stationed