This bothered me for the longest time too....one of the coolest looking villains in the franchise was given a couple of brief lines then sliced in half to (apparently) fall to his demise. It wasn't until someone turned me onto the Clone Wars that I discovered he didn't die, and that his backstory (like the fact he had a brother),relationship with Palpatine, and ongoing ambitions were treated with great thought in both that series and forward into Rebels. As I mentioned before, he had a long and interesting life after Phantom Menace and his role in Star Wars canon isn't done yet. He won't return in the films, but he has proven to be a much more complex villain in the two animated series then he ever was in Episode 1.
Lotsa Maul in Rebels