Not exactly changing anything, but making the ending better:
First, I would cut Episode 8 entirely, which means Jack goes to the Guardian's place right after getting his sword in Episode 7. With this, I would make Episode 9 end right after Jack and Ashi travel through the portal, allowing the episode to be better paced and making a great setup for the finale. Not to mention the "power of love" bullshit that makes Ashi break free would be more justified and impacful, since they would not have confirmed their romantic feelings for eachother until that moment.
So, in the final episode, we get a longer battle between Jashi and Aku from the past, and Ashi actually dies right after Aku is defeated, since she's part of him (yeah, much better than just vanishing from existance days after the battle). This saddens Jack, who's still in deep pain after he returns to his land and frees his people. He goes to mourn in the florest (having his conscience taunt him would be a nice plus), but gets over it after seeing a ladybug, and the scene goes on just like in the episode we got.
But, instead of closing to the credits, the camera pans upward and we see a fast-foward sequence showing the world changing as the centuries passed, while quickly showing pictures of specific events of the world's history. But, as it gets farther from the current times, the world starts looking grim, showing a world not that different from Aku's future. Not by the power of a Demonlord, but from endless wars/scarcity of natural resources/whatever reason you can think for a distopic futuristic world.
Then suddenly, it all turns to black. and we hear in Jack's voice something along those lines:
"Many years have passed, and I still haven't find the one who cursed this land. Aku's grasp chokes the past, present and future, and only I can fix this. Gotta get back, back to past..."
And the camera pans upwards showing King Jack, exactly like the Guardian's Vision.