Just imagine it. Jacks on his knees, he's given up and about to be defeated by Aku. When suddenly, rumbling is heard in the background. Jack opens his eyes to see an army approaching. It's lead by the Scotsman ghost, and it's filled with all the past characters jack has helped save.
A battle ensues. Jack is somehow reinvigorated, he can't give up on the future now. He manages to get his sword back, and goes after Aku.
Ashi will once again stop him, but somehow he'll save her (my guess is that the sword will only harm the evil within her)
Then with Ashi freed, he'll finally destroy Aku.
This is when he'll come to the sad realization that he's failed in his mission, and will never be able to return to the past. This will be temporary however, as he'll realize that Ashi is what he really wants, and that the future can be fixed.
Jack will be made king for saving the world. It will then cut to scenes of him living out his days with Ashi and the world being renewed. He'll age now that Aku is dead.
Cut to a scene of him when he's old, perhaps grieving the loss of Ashi. When suddenly a messenger runs into the room and hands jack a note. He'll look up with fire in his eyes.
The scene will then cut to pic related. Jack on his horse, wearing his crown and looking down on the last portal, which has been hidden all this time, the guardian standing right in-front of it. The End.