Snoke is Darth Plaguios (sp?). Mentioned in the prequels by Sideous to Anakin at the opera music show thing about how he discovered immortality. Sure he might have got knocked over the head in his sleep by Sidious, but he's immortal right? Would explain the massive scar in his head too. He's been waiting in the shadows regaining his strength, letting Sidious do all the hard work.
Also, Luke. He's standing at a grave at the end. I'm positive that's a tombstone. Who, his wife? How did she die? That make Rey his daughter, was she hidden from whoever killed his wife maybe?
Don't reckon Rey is going to the dark side, she is angry as hell with a lightsaber though!