my first idea to explain this seemingly ancient and extremely experienced bad guy was that he isn't actually there - that the reason he only appears in hologram form is, very simply, that that is all there is. In other words, some kind of very advanced holocron created by some Sith of the past, which is fully interactive and has a memory and the ability to process new information and everything.
I could be completely off-base and violating half a dozen established canon rules with that theory, though, I'm far from an expert.
Your theory about Darth Plagueis seems plausible enough, but the main argument against it is that, as per my five minutes of research just now, Snoke's appearance was left to the very final stages of post-production to decide. Unless you figure they just said that to throw people off.
My favorite theory is that he is a Sith Holocron but DomA read the book adaptation of the movie and he says that both Han and Leia know of Snoke. I'm not sure if he confirmed it but I think he suggested that they have met him?
It's also wierd that Snoke wants Rey to be brought before him in person if he's a holocron. Then again, Sith ghosts have real power esp on Korriban so who knows.
Fan of Lanfear