I agree with Kot, that tacking big things onto an otherwise complete saga never quite gels properly. Beat the big bad? Now there's another even bigger bad that nobody ever hinted at before. that sort of thing can in fact introduce 'retroactive' plot holes.
If there was another Sith Lord around then it clearly breaks the Sith principle of the "Rule of Two". So Snoke could exist, however he has to be a "dark force user" who is entirely unconnected to the Sith, which itself brings in all sort of questions if there are otherwise unrelated Sith-like people just hanging around who don't have the convenient "Rule of Two" of the Sith which is there to help explain why they don't end up with armies of Sith. The real question is whether he gained force powers after Palpatine's death, becoming a "fake Sith" or whether he had those powers already, which begs the question of why Palpatine wouldn't hunt him down just like he did with the Jedi. I doubt the Sith with their strict "Rule of Two" would allow any other force-sensitive person to exist, light or dark.
So the only real believable thing would be that he's some random force-power using person who happens to be on the Dark Side, but was hiding from the Imperial forces during the Empire period just like Obi Wan and Yoda were, and the reason he acts like Palpatine is because he's a newly minted "Sith wannabee".