I thought I had written this all down, but I guess not. Here is how Snoke will break down (imo), and why. This contains summaries of relevant portions of Aftermath, Lords of the Sith, Thrawn, Tarkin, and other works.
1. He will be a new character, and not someone from the current extended canon or the old EU. Brand new. Why? Because Lucasfilm has shown over and over again that they understand that it cannot pander to its superfans. The movies must stand totally alone. The TV shows, books, and comics must all be completely superfluous.
2. He'll be an alien. That's confirmed. I'd be shocked if he is not a NEW alien.
3. He'll be a Dark Side user, but not a Sith. Lucasfilm execs aren't dummies. They aren't gonna undo all of the mythology and story of their (admittedly ridiculous) "chosen one" arc for Anakin by having more Sith out there. Yes, it's shaky even with other Dark Siders around, but the Jedi-Sith struggle is done for now. Will we see a resurgent Sith sometime down the road? Absolutely, but not for quite a while.
4. He will be from the Unknown Regions. The short version of this is... duh. That's where the First Order was born. But there's more to it than that, about which I have to expound a bit.
The Emperor's Grand Plan
Prior to the battle of Endor, the Emperor was unable to see past that fateful day with any confidence. There was a shatterpoint in his farsight, and while he had done everything possible to assure his victory, he knew there was a chance that he, and the Sith, would be destroyed. So, he took steps.
Prior to the battle, Palpatine sent his protege, Gallius Rax, to the wilds of space with a Super Star Destroyer and some escort ships. His mission, should Palpatine die, was to take control of the remnant. He was not to rebuild the Empire, but was to burn it down. Leave only the hardest, most fanatical core, and then retreat with them to the Unknown Regions and start anew. Waiting for him would be more Imperial forces that the Emperor had arranged to have in hiding, hand-picked to survive the scourge.
The Unknown Regions
So, what are the Unknown Regions? This is not the deep, mysterious edges of the galaxy. Wild Space is the term for the extreme borders of space. The borders of the Unknown Regions are known. It is a pocket of known space that is nearly impassable. There are no known safe routes through it. It's full of bizarre phenomena and storms and alien creatures that live in empty space. One exception is that Ilum, the traditional source of Kyber utilized by the Jedi, was reliably reachable based on ancient Jedi knowledge... the Empire was also able to exploit the Jedi's route to Ilum after the fall of the Republic. But penetrating deep into this part of space was known to be impossible.
Once inside its borders, it is apparently livable but still incredibly dangerous. This is where the formidable Chiss Ascendency is found, the empire from where Thrawn hails. The Chiss are constantly assaulted by incredibly dangerous forces that made the Empire under Palpatine look like nothing. "Pure evil," as Thrawn describes. It was seeking allies against these forces that led the Chiss to send Thrawn into Imperial space. Alternately, if the Empire were not found to be allies, the plan was to try and draw the Empire and the evil forces in the Unknown Regions into war with each other. Whatever is out there hounding the Chiss, it is no joke.
Palpatine and the Unknown Regions
Palpatine was obsessed with the Unknown Regions. He wanted very badly to get there.
For one, he was never satisfied with an Empire that controlled only the known parts of space. He wanted to control the entire galaxy. And other galaxies. The Unknown Regions was the next step in his plan of conquest.
Second, according to Palpatine, inside the Unknown Regions can be found the source of the Dark Side of the Force. The Dark Side emanates from a particular place... it is not created by life as the true Force is. The Living Force comes from everywhere (though it does have an origin and is stronger in some places than others). The Dark Side, from somewhere (according to Palpatine's understanding) in the Unknown Regions.
It was the Emperor's obsession with this part of space that led him to ally with Thrawn, to try and gain intelligence about the region and learn how to traverse it. It is why he set up an Observatory on Jakku to try and plot a course through the Unknown Regions, which eventually succeeded.
So, Supreme Leader Snoke...
The hardened and honored remnant of the Empire selected by Palpatine and Rax left known space and headed into the Unknown Regions, where it met up with additional forces hidden by the Emperor after his death. Rax was dead by that point, and Sloane in command. Brendol Hux (father of Armitage Hux that we know from TFA) was brought along to train the kidnapped and brainwashed children that would form the core of the First Order's fanatical forces. The Emperor's own SSD was waiting in the Unknown Regions.
Something happens, though, that takes things off track. Between the end of Aftermath and the start of Bloodline (prior to TFA), the remaining pieces of the Empire are presumably taken over by Snoke. It seems that he has been keeping an eye on the known galaxy. His First Order curries favor and has political influence in the New Republic by proxy. He lures in Ben Solo. He creates the Knights of Ren. He commands the absolute loyalty of dyed-in-the-wool former Imperials like Armitage Hux.
My thought is that Snoke will be a powerful alien Dark Sider, possibly trying to gain access to the wellspring of the Dark Side. The Awakening in the Force has somehow interfered with his long-standing plan and he is using the First Order to try and snuff out the resurgent Light Side before it gains a foothold. I think that by the end of TLJ, and certainly by the main act of Episode IX, our heroes will be in the Unknown Regions fighting the battle there. I think having a whole new area of the galaxy to serve as a setting for exploration and adventure will provide a lot of fun for the next episodes in the saga.
