My problem isn’t that the Emperor isn’t new, it’s that it’s reductive. The film literally tried to make him the only evil that’s ever been. He’s ALL of the Sith. He’s the major villain of all three trilogies.
I would’ve been down for an evil Luke as the villain for the sequel trilogy, he wouldn’t be a new character, but he would be a new villain. Similarly, Jar Jar would’ve been a new and creative choice.