If Darth Maul makes a return it would likely be through a training droid (something like Proxy) or that a "Maul" is more of a clone sort of deal and that it's not the real one. Though I could imagine First Order cloners making a Maulkiller for reasons (always stuck me as odd we never got a full story for that). A training droid would be more likely though since it would make sense for Kylo Ren to have something to spar with if he had to fight a resurgent Jedi order.