The short answer would be von Sydow aka the ancient fear. Fingers crossed for Marka Ragnos. This is kinda the problem if the same story is followed with the same people being the good guys. If you defeated the empire and killed the Emperor, who would be a credible threat? Sooner or later the reset button will have to be pressed(everyone you know dying of old age and a group of unrelated people fighting a new evil about as threatening as Jar Jar). But anyway, I would love to see Ragnos, but from the Movie Universe standpoint, it is not likely. Only two sith are named in the saga that died before the movies. Plagueis and Bane. I'm leaning towards Bane. First, the plagueis novel that was very much endorsed by Lucas revealed that Plagueis was not a legendary figure but someone very contemporary. Furthermore if they are the remnants of Palpies inquisitors, they'd probably be vary of Plagueis. Bane however is awesome, legndary, over a thousand years old, was shown onscreen, and has a story that doesn't need any retconning.