I think Snoke is either this guy (although I don’t watch the Rebels show, so maybe it’s completely impossible, dunno) or Darth Plagueis (using mainly the OST as evidence).
That guy got killed dead in the season 1 finale. Killed real dead.
OK, so Plagueis it is. Apparentely when Andy Serkis was asked why Snoke needed to be CGI, he said it was because they needed him to have a hollow face. Now we know the character design was only finished quite late in the production, but the hole in his face was firmly set. So it is probably really important for the story. Perhaps Plagueis has been this deformed since his apprentice killed him in his sleep by breaking his face…