If you find yourself in the situation where a major ongoing villain is actually irrefutably dead, so you can't pull the trick of bringing them back for a dramatic new evil plot, the next best thing is to introduce a new villain with echoes of the old one.

The new villain can be a relative of the old one, or a lover, or a protégé, or—to borrow from our own work—a clone, or an artificial intelligence copied from their brain patterns. But the players don't have to know that at first. Let them see the old patterns and modus operandi of the villain they defeated, without revealing who it us they're up against. At some point they'll start to get paranoid and wonder if the villain wasn't really dead after all. That's when you spring the surprise on them.

Transcript

{Robin, who looks like a small frog, appears backstage}
Robin: Uncle Kermit, Uncle Kermit!
Kermit: What is it, Robin?
Robin: I was sitting halfway down the stairs when I intercepted a radio message. A Sith Lord has raided Kashyyyk and fled with a prisoner to Koozebane!
C-3PO: Kashyyyk? Chewbacca's there!
R2-D2: A Sith Lord?? I thought Vader was the last one of those. Or possibly the only one, ever.
Robin: I just tell 'em like I hear 'em.
Luke: Finally some action! We go to Koozebane, fight the Sith Lord, and rescue Chewbacca!
Kermit: Wait! Need me, you will!
Luke: Oh, not like last time when you let me go to Bespin alone, then?
Kermit: Turned out so well, that did not.