Will be played by Andy Serkis. It seems confirmed he's a Mo-Cap character. He's going to be the ruler of the First Order, what remains of The Empire. And it also seems confirmed will be a Dark Side Force user and Kylo Ren's master.
I was really hoping that the remnants of The Empire and the Sith or their replacement would be separate from each other. After all, we saw in the Original Trilogy that the purely military people in The Empire weren't exactly happy having Vader and his religion around. That would have made for a more complicated scenario, but whatever.
There has been some speculation about Snoke, one rumor is he will be some kind of Snake or Reptilian alien. If that is what happens a lot of Conspiracy Theorists are going to have a field day with it.
Whatever kind of Alien he is. I have a hunch that he may turn out to be Darth Plagueis.
If what this Trilogy is going for is making Star Wars into a sort of Trilogy of Trilogies, then that is would be a good way make them interconnected. A villains who was mentioned as an important plot device in Episode III but never seen.
One of the main things we know about him is that he supposedly learned how to cheat death. So revealing he wasn't as dead as Sidious thought would fit.
Remember the old EU isn't canon anymore, so nothing about the old Darth Plagueis novel has to be stuck to. Including whatever species he was said to be there.