I'd rather have cool new characters if it's an option to do so. Yeah, maybe it'd be cool for Serkis to play Plagueis. But I think it's even cooler that he's going to create his own character, unique to this movie.
New characters keep things vital in a way callbacks can't.
Plagueis would have effectively been a new character anyway since he was so sparsely used. Due to the canon wipe they could do whatever they want with him, opening up a whole new story while also tying everything together with a nice bow. The circle <would have been> complete. It depends on how Snoke turns out though - if his story is really interesting and compelling, there won't be a problem. If he's just an ex-Inquisitor or something, well, that's kind of boring.
I think part of the desire to see Plagueis reappear, or at least how it was for me, was that his death was left open. Did he really die or did he somehow find a path to immortality after all?
Charles Foster Kane said:
Sometimes I feel like a noob. Why the Sith never coming back? Isn't a Sith just a force user who goes dark? Wouldn't a "knight of ren" force user be essentially the same thing as a Sith?
The Sith are a distinct Force tradition that severed ties with the Jedi during the Hundred Years' Darkness. They have their own views, rites, traditions, etc. It's like how just because someone uses the Force and isn't bad doesn't mean they're a Jedi - the Jedi are a specific organization. Similarly, in real life, just because someone believes in Jesus doesn't make them Catholic. You have to follow the teachings.
The Sith were reorganized by Darth Bane following their collapse after the Jedi's victory in their final war after the fall of the Old Republic, establishing the Rule of Two, in which there could only be a master and an apprentice. Since Anakin killed Palpatine and got himself fried in RotJ, there were no more Sith left. If someone then called themselves a Sith, they would be a pretender (unless, of course, they became so successful that nobody else cared and pretended it was a Sith revival), because there would be no legitimate claim to the chain of previous Sith. This was all much more haphazard in the old expanded universe, where the Banite Sith were merely one branch of Sith who survived after the Seventh Battle of Ruusan, with others surviving on Kesh, Kalakar Six, etc. and other branches of the Sith popping off of the Banite Sith or rising in opposition to its ideas (Lumiya's Sith, the One Sith, etc.). None of that extra stuff is canon anymore though. The Knights of Ren appear to be a totally different order of Force users who share some aesthetics with the Sith, but what their purpose or world view is like is so far unknown.
