So Maul of course escapes into hyperspace at the end of Clone Wars, eventually returning to Dathomir and leading Crimson Dawn. This also leads somewhat to his cameo in SOLO. Although it's nearly a decade after the events of Clone Wars, it's safe to assume Maul did largely the same thing he did in CR, although with the Empire around instead of the Republic, and Palpatine probably more focused on other things than keeping Maul in line (Would be nice to have a Vader vs. Maul thing though).
The big guestion is what happened between Solo and the second season of Rebels, another eight years. Maybe with Vos gone, and perhaps Jabba moving up in the underground (Although didn't Ezra use his Jabba nickname on Maul and he didn't recognize it? I might not be sure) he decided to check on some old Sith secrets and got stuck on Malachor, where he's found in Rebels, although it's unclear how long he's been there.
Wonder if there'll be some kind of followup on this. I'm not sure a Maul movie would work, Ray and Witwer are good and all but I'm not sure that'd carry a whole movie. I'm thinking perhaps a novel or Marvel comic series down the line is probably the most reasonable extension. A TV show I guess could also work out.