I think it could be a flashback style movie, so it could include Maul. The problem is, what's the point of Maul? Maul was interesting as he was solely a physical threat to the Jedi, a threat to their lives, and a threat of destruction to their order with the return of the Sith Lords. There's nothing more then that, so I don't see much of a reason to go that route again as it's not a philosophical battle. Most good stories involve those philosphical battles (Episode 3, the entire original trilogy, R1 to an extent, I try to forget that movie), and Obi-Wan, the sage archetype, should have that as the central part of his movie. Maul just, wouldn't be able to provide that.
That said, we still don't have a good answer for what would the movie tell, only that it should include a philosphical battle.