I've been working more on this lately. For the most part or completely, I think Palpatine will remain in the background. To the extent that he is seen, it will not be as a threat. He is quiet, meticulous, careful, respected, a natural choice to take the top slot when the time comes. He will look like himself, just younger/less creepy - so that his later appearance is accepted as due to advanced age and decades or evil magic, but not entirely startling. There will be the uncomfortable gap between his apparent goodness and his less friendly exterior. It's all about the seduction of the Dark Side.

A main villain will be a Darth. Very charismatic, idealistic, an undercurrent of anger but not sneering and saying evil things - in other words, what Dooku should have been. He will be young (~40). Again, it's about seducing Anakin to the dark side.

And then there will be Darth Vader. A force of great power on the battle field. Nobody knows where he comes from.