In the case they really would want to have some Sith in this movie (a thing that I think is not needed, but whatever), my guess is they would think about using Darth Plageis. As a ghost, or he maybe never died as Palpy thought. I think about him simply because he is the only Sith ever mentioned in the movies that we never saw on screen (there's a couple of drawings and even some action figure I guess but I don't know how "canon" they are?). Using him would be a bit unoriginal (and would once again remove any mistery around a SW character) but it would be the only way to tie the sequel to something we already know about the Sith. Of course you can make a new trilogy without anything tied to the past movies. But if you're using some original cast and still using "Episode" in the title, then it needs to have some connection. And if we speak about Sith (IF there are Sith in this movie at all), then Darth Plagueis is the obvious option (I'm not saying the best, only the obvious)