Could Sifo-Dias be revealed as Darth Plagueis? I mean he ordered the creation of the clone army (whether it was Dooku pretending to be Sifo-Dias or not is not explained to my knowledge so it's open for interpretation). Sidious doesn't mention when he killed his master. Sifo-Dias was presumably killed around the same time Anakin met Palpatine. And if by chance that is true, then it would fit in nicely and offer continuity - something like a legacy of evil - the perfect adversary for the Skywalker legacy. It would make Plagueis seem like a menace, surviving Sidious' assasination attempt, keeping a low profile and attempting to rise to power once his old apprentice is no more. And with that there would be no need to clone Palpatine or anything like that, since the main villain would be connected to the previous events. That would have a more powerful impact on the story rather than introducing someone completely new.