Argh. Had an Idea!

It has been previously suggested that Sidious is replaced by Dooku in TPM, so the assumption is that Dooku is the Master, and Maul is the Apprentice (following the rule of the two).

Anakin kills Dooku at the opening of ROTS, and it is widely assumed at that point that the Prophecy of the Chosen One has been fulfilled.

At the Jedi Council, continues to sideline Anakin making him feel redundant and impotent. The War is over, the Sith are gone, Anakin has fulfilled his destiny, surely the Star Wars Trilogy is over right? Wrong!

In a big twist Palpatine reveals that he is in fact the Sith Lord, and he had many apprentices.

Not sure how Palpatine could make an effective argument for Anakin to turn to the dark side though.