Warning: Spoilers for 'Star Wars: The Force Awakens' lie ahead!
THEORY: Supreme Leader Snoke is the new alias of Boba Fett, the same way Darth Sidious was the Sith name of Palpatine.
Here's how I came to suspect the true identity of the new Star Wars master villain:
From his limited appearance, we can see that he is a scarred human. The last time we saw Boba Fett, he was being digested in a Sarlacc Pit. Digestive stomach acid and teeth would leave Boba looking just like Snoke does. Fans have always wanted Boba Fett to return and Disney may have listened.
A big clue that Boba is alive is the scene in Episode 7 where Poe Dameron's ship sinks in the sand. Is this to remind us of Boba being eaten by a Sarlacc? A clue? Like Fett, somehow he survives. Also this makes Poe seem like a spy how he doesn't go into detail at all about surviving. The filmmakers tell, but don't show what happened to him. When Finn asks Poe for help rescuing Rey, Poe takes him to Leia instead of helping Finn. If the writers were building a true heroic character in Poe, he would've helped the same way Han chose to help Luke & Leia in Episode 4. This connection to Boba perhaps shows Poe was saved by The First Order.
What would this mean? That Kylo Ren is being groomed not as a Sith, but as a Mandalorian who can use The Force. And Kylo's murder of Han Solo would not just be out of the scorn of a son, but revenge for the disfigurement of Boba Fett by Han in Episode 6. Boba having been an ally of Darth Vader would very much attract the dark side in Kylo.
"Even you, master of the Knights of Ren, have never faced such a test."
―Supreme Leader Snoke to Kylo Ren
Remember that Jango Fett put his son Boba through grueling tests to become a Mandalorian warrior. I imagine Snoke has done the same to turn Ben Solo into Kylo Ren. Also, notice the similar design in Kylo's mask with Boba's and Phasma's suit with Boba's.
If I'm right, then the next films will have lots of Mandalorians. And the Knights of Ren may be a group of Mandalorians who act under Snoke's control. They are obviously not Sith because only Kylo seems to have the Force to wield a lightsaber. We'll find out in a few years.
EDIT: Also, I'm reminded that Boba Fett returning was a part of the Expanded Universe. Disney resurrected the concept of Darth Caedus with Kylo Ren, so the idea of Boba returning looks even more favorable.
EDIT: So the biggest hole in my theory is that Snoke is Force Sensitive, having felt a disturbance in The Force, and Boba Fett has never been confirmed as Force Sensitive. So I did some research and found this debate on a Boba Fett forum. Some consider him Force Sensitive from the amount of Midi-chlorians that would be in the remarkable DNA he received from Jango Fett and his sheer toughness in the Star Wars universe. He is not strong enough to be a Jedi or Sith though, which could explain why Snoke is neither. Again, the official canon has never said Fett is Force Sensitive. But its never been said that he wasn't. Perhaps if Boba was discovered by a Jedi as Anakin was, he would've reached an even greater power.
EDIT: Too many rude, crude and malicious comments have made me ignore whatever conversation is going on below. The fact that Snoke is 7 ft tall is very interesting and doesn't help my case, but its not enough to dispel the idea of Snoke being a deformed Boba Fett. Why? Artificial limbs and/or an artificial body. If Disney wants to bring back Boba as a new character, they will do it and trick most fans doing it.
EDIT: People say Boba Fett can't have The Force. Remember that Luke and Leia didn't know they had The Force until they were 19 and Luke still became a great Jedi (as did Leia in the Legends canon).