I guess there are two possible extremes to go, with the villains.
After the extremely powerful and sinister Emperor of the prequels, they could go with The Even Badder Dude Behind Everything, or all the way down to a pretender affecting it all. I'm not a huge fan of the former just because there's surely a limit to how many plausible levels you can go with that one, and I don't think it's necessary for a story. The heroes defeat the Ultimate Evil, only to have to defeat the Ultimate, Ultimate Evil later? This is the Final Ultimate, right?...
It would also make this new series dundundunDUN, it was Darth Plagueis!...which could be really cool, I suppose, or a letdown. It seems like too little new discovery to me. Also, too much of building the arc on foundation laid by the prequels.