When setting up antagonists, try to have a selection of different villains, to keep the heroes guessing. Imagine if Batman only had The Joker to deal with. Museum robbery? The Joker. Hospital explosion? The Joker. Bank heist? The Joker.

Boring, right? And it reduces the need for Batman to be a master detective.

While it's good to bring back arch-enemies, you have to intersperse them with other nemeses to make the recurring master villain feel special. Each new adventure should leave the players guessing who's behind it this time, and if it does turn out to be the arch-enemy, it'll make the adventure feel extra special.

Transcript

R2-D2: Oh my god, I've worked it out. The clones that don't look anything like the originals. The pigs on Endor. The song and dance number. It all makes sense!
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Luke: Go on.
R2-D2: It's Nute Gunray!
C-3PO: Of course! Obviously!
Chewbacca: Obviously??
C-3PO: There aren't any other real NPC villains in this universe.