Mako voiced Aku. He also voiced the Uncle Iroh character on AVATAR: THE LAST AIRBENDER until he died; he was replaced by his protege Greg Baldwin, who studied under the voice actor and does a damn fine job imitating not only his voice in general, but his timbre, pacing, and style:

www.imdb.com/name/nm2055652/

Greg could easily pick up the mantle of Aku.

That said, Tartakovsky — in just about every interview he does — speaks of a SAMURAI JACK movie that would end the series. Mako’s death may have provided an excuse for another delay, but truth be told, Tartakovsky’s a busy man. Right now, I believe, he’s doing the pre-production work on “The Power of the Dark Crystal” (not sure if it’s a continuation, a remake, or a reimagining).

But he always gets asked about Jack, and always talks about Jack.