I so wanted the guardian and his time portal to come back as a later plot point, but not like this! Not like this!! sob

Tinfoil mode go:
I've stated a theory before, and it goes like this. Jack isn't the one who will defeat Aku. His son will. I would have said son or daughter but image above.
Remember when Jack got his sword back, how he was deemed worthy as his father before him? And when he became worthy, the sword teleported from wherever it was to him? Good storytelling always works in threes. First the Emperor gets the sword, then his son, then HIS son.
Aku doesn't kill Jack, he just throws the sword into a black hole or something and lets Jack suffer because Aku is more evil than he is effecient. This is a know personality trait of his. He probably releases Ashi because reasons. Time passes, he has a son with Ashi, who becomes worthy of the sword. The Scotsman's grandchildren form an army, they all defeat Aku, defeats the Guardian, and goes back in time. Anything else would leave loose ends.
Remember how we see the samurai return to the past only he looks older? Well, that isn't Jack because Jack doesn't age. Its another samurai.