Count Dooku
Now that this character has been confirmed, will the Jedi believe that Dooku is the Sith Master that Yoda and Mace were referring to in TPM? It makes sense, he's old enough and supposidly talented enough to train an apprentice like Darth Maul. Then when he is killed in EP III the Jedi could think the threat is over, then Palpatin could spring the surprise...