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.