At the end of the game, Aku is slashed by Jack and Aku says "This is not over..." as he turns into a dark cloud. Jack then looks where the dark cloud was and then Ashi appears and you know the rest. I don't think Aku was killed, as I don't think jack would have looked into the sky, just as he always does when Aku is getting away as Jack watches. I also don't think his voice would linger that long, as his voice was clearly heard after he was turned into a cloud. There wasn't a 100% confirmation that Aku was destroyed.
To me, this is a huge gray area for the series. Aku might still exist in this pocket of time, or whatever is left of it, despite it collapsing. It is POSSIBLE Aku still exists, just in another time, and might one day return and perhaps the series might continue, with Jack + Ashi as the main characters.
Gendi has talked about jokingly that maybe Samurai jack is his mickey mouse and maybe should have gave the series more attention.
All I am saying is that I think it is definitely possible that Samurai jack might continue, whether it be another games or a series again. It might also explain why now Ashi surviving might be the canon ending, as Gendi has ideas for the future of Samurai jack.