BEFORE YOU SAY ANYTHING:
Let's forget about the expanded universe for a minute...
Focusing on the movies...
In The Phantom Menace (the movie!) we know very little about Darth Maul. He's Sidious' apprentice. That's it. He is a mystery.
Now In Episode II we learn A jedi Master named Sifo Dyas ordered the clone army. Sifo Dyas died ten years before Attack Of the Clones so he died during the events of The Phantom Menace.
Now...
Skip to Episode III. We know that Palps tells Anakin that his old apprentice was Jedi Master Sifo Dyas, the one to order the Clone Army. Sifo Dyas died during The Phantom Menace. He died before Count Dooku joined up with Sidious. He died when Maul died.
So...
Remembering what Yoda said about "Only two there are" doesn't it make sense that Darth Maul is a fallen Jedi?
Maul is Sifo-Dyas.