Not sure how they'd do it, he's finally killed by Obi during Rebels which takes place like a year before the events of A New Hope and The Mandalorian takes place 9 years after those events. With all the necromancy fuckery of the new films maybe they'll go that route, or he could just have a cameo in a flashback type arc in the show.
One of the main sources of the rumour is that the actor for Maul from The Phantom Menace and the voice actor for Maul from the animated series both posted stuff on Twitter hinting at season 2 of The Mandalorian, because Moff Gideon was in possession of the Darksaber which is a Mandalorian Jedi weapon that many characters have been in possession of, Maul included.
Kinda hope they don't revive him in some gay Disney-tier fashion, but a series of flashbacks could be cool.