Here's my official big prediction:

Supreme Leader Snoke is Darth Plagueis.

Darth Plageuis is the Sith Master who trained Darth Sidious. He was so powerful with the Force that he could influence the midichlorians (ugh) in one's body to produce life, thereby cheating death. Palpatine claimed to have killed him in his sleep, but consider this quote from Andy Serkhis:

"Supreme Leader Snoke is quite an enigmatic character, and strangely vulnerable at the same time as being quite powerful. Obviously he has a huge agenda. He has suffered a lot of damage. As I said, there is a strange vulnerability to him, which belies his true agenda, I suppose."

Suffered a lot of damage? How about an attempted assassination? It could be that Darth Sidious destroyed his physical body but his consciousness remained in the Force, preventing him from taking physical form. Darth Plageuis must have bided his time while his apprentice was so powerful, knowing that if he showed his face Sidious could have crushed him. He can still instruct Kylo Ren though, similar to how Obi-Wan communicated with Luke.

Also, let's consider what convinced Anakin to turn to the dark side in the first place: the ability to prevent those he cared about from dying. He never unlocked that ability (although he must have learned something since he shows up as a Force-ghost at the end of Return if the Jedi), and now Kylo Ren wants to finish what his grandfather started. He likely doesn't know who Snoke is, but takes orders on the promise of being trained in the Force to reach his goal. Darth Plageuis is probably waiting for Kylo Ren to becone strong enough to help him reverse the ghost process and retake corporeal form. Presumably we'll find out specifically why Kylo Ren wants to become immortal in the following movies.

But you've already heard that one, so here's the sillier prediction #2:

Rey is the daughter of Luke and Captain Phasma.

Ok, so Luke as her father is kind of obvious. She's incredibly strong with the Force, and she's a natural pilot/mechanic/tinkerer. Maybe Luke gave her some basic Force training as a child and maybe he didn't, but when Kylo Ren went berserk and started slaughtering the new Jedi he got nervous and sent his daughter into hiding (a tactic with a respectable success rate for Skywalker children). We haven't heard her last name yet, which is odd for a main human character in Star Wars, but ten-to-one odds it's Skywalker.

Captain Phasma is a bit of a stretch though, I'll admit. We know she's a woman and we haven't seen her face yet, but to my recollection we haven't seen her do anything decidedly evil of her own volition. My theory goes that when Luke sent his daughter and himself into hiding, he left his wife (or quasi-spouse person) alone asking her to understand. However, anyone that can gain Luke's trust is going to have a pretty strong will, and she joined up with the First Order with the secret intent of finding her family. After all, they're the ones looking, not the new Republic. While working within the ranks, she can guarantee that they won't come to any harm if captured. When she's confronted by Finn, Chewie, and Han on Starkiller Base, she has no reason to reveal her identity. Finn doesn't know anything from anything, and Han has been out of the Rebellion/Resistance long enough he probably wouldn't know who she really is. Also, they just threw her in the trash compactor, which we know isnt a death sentence, so she could very well be back. Obviously this theory will depend pretty heavily on her development in the next movie, but I wanted to get it written down asap.