Whilst I don't favour any of them returning, I'd say Windu or Fett surviving their unfortunate film endings would be more plausible than Maul. The only way it would be plausible for Maul would be if they went with the absolute truth angle. He turns up and says, "I was just too cool to die." Then he says "Eeeyyyy!" like Fonzie, gives the thumbs up and rides off on his Sith motorbike thing... in a leather jacket.