Well we as the viewers know a lot more about Luke's character than Palpatine did. Palpatine was evil & had an evil worldview. He was overconfident & thought that everyone was corruptible. If Luke was a different person he could have turned him, but ultimately Luke was good at heart & embraced Yoda's teachings.

Anyways, to the subject at hand. I think the bad guys should be Mandalorians. Not to demystify Boba Fett's character even more, but I think it would be a cool way to bring something new that's connected to the other trilogies without rehashing old plotlines or bringing back old enemies.

They show up en masse from out of nowhere with a powerful ruthless leader (Sydow) & a couple of young brutal self-tough dark Jedi. They want to rule the universe and they think that establishing a new order dark Jedi that would be submissive to their Mandalorian masters & thus more stable would them rule the galaxy, thus they target the next generation of Skywalkers for purposes of breeding them with with their dark Jedi to create this new order, thus providing reason for the original cast to be involved in the new saga.

Boom. No long lost sith or imperial remnant, but new enemies and new dark jedi that seem old and familiar.