I don't get why there is a controvery surronding what was carried over from Legends. Is going on Wookiepedia and just looking it up so hard for so many people ?
We haven't seen any Dark Side user besides the Sith on the big screen. For the casual Star Wars fan it would be something new.
Pablo Hidalgo, Creative Executive of the Lucasfilm Story Group, confirmed on Twitter (Source) that Plagueis was killed by Palpatine. Snoke being Plageius would really be even more unoriginal than Starkiller Base, seeing how it's the number one theory for who Snoke really is.
Just saying that if Snoke really was Plagueis and it's supposed to be a big reveal, I doubt Pablo Hidalgo would come out and admit that Plagues was not in fact killed.
As far as Snoke = Plagueis, it may be the number one theory, but the other theories are garbage. I doubt Snoke's identity will be that big of a reveal anyways