I recently read a theory that Snoke is none other than Darth Maul. While I personally think this theory is far-fetched it got me to thinking what if we've never technically 'met' the character before but we know who he is? The one theory I have not seen floating around (and if it's out there I hope it gains more traction), is Snoke being none other than the elusive Sifo-Dyas that we hear about in the Prequels.
Here's why I think this may be the case:
In Attack of the Clones we learn that a decade before the events in the movie unfold Jedi Master Sifo-Dyas commissioned the Kaminoans to build a clone army. It isn't until later on in the Clone Wars animated series that we get a little more backstory. Being gifted with the power of foresight he saw the events that would lead to war in the galaxy leading Sifo-Dyas to argue that the Galactic Republic needed an army. The Jedi Council thought this was a radical idea and had him removed from the council, yet he still went through with his plans in secret and had the Kaminoans start work on the clone army.
Before I go any further in unpacking this idea, I know that many of you are thinking 'but in that same arc it was explained that the Pyke Syndicate killed him on Dooku's order'. And while that could be the piece that debunks this entire theory, all we know is that his ashes were delivered to Dooku. It's plausible the Syndicate didn't find Sifo-Dyas' body but pulled a fast one on Dooku, and Sifo-Dyas did indeed survive the ambush of his shuttle.
So what we know, is that Sifo-Dyas foresaw Galactic turmoil and was willing to, at the cost of his seat on the Jedi Council and loss in stature in the order in general, argue for and pursue the building of an army to protect order in the Galaxy. For his effort he's left for dead by the Jedi and the Republic that in his mind he was trying to save. His plans for a clone army are then hijacked by the Sith, leaving him no love for the emerging Galactic Empire.
What we know of Snoke is that he, in the Unknown Regions following the collapse of the Galactic Empire, rebuilds a new order out of its ashes to once again restore order to the Galaxy. We know he witnessed the rise and fall of the Galatic Empire and clearly has knowledge of the Force. And also he's not very nice to look at.
I believe that Sifo-Dyas survived the attack on his shuttle over Oba Diah, though it left him severely disfigured. Feeling betrayed by the Jedi and the Republic, and still opposed to the ideals of the Sith he retreats into hiding carrying with him a lot of resentment and anger that leads him to fall to the Darkside. His motivation is not power, but order and peace in the galaxy at all cost, hence why he doesn't side with the Sith and instead falls back to the shadows to watch and learn. With all parties in the galaxy thinking he's dead it would make it easier for him to quietly observe and plan to see his objectives come to fruition. Snoke's desire to wipe out the last Jedi and restore order to the Galaxy is not born of some evil villain color by numbers book, but instead driven by his view that the Jedi's shortsightedness is the reason the Galaxy fell apart in the first place coupled with what he viewed as a corrupt and blinded Republic.
In closing, I think that Sifo-Dyas would have enough anger in him to fall to the Darkside, but not see himself aligned with the Sith, instead he's motivated to right the wrongs in the Galaxy he lays squarely at the feet of the Jedi and Republic he once tried to warn and save. He doesn't view the Jedi as guardians of democracy, but a problem and old way of thinking that needs to be eradicated. He doesn't identify with the Sith's ends, but certainly feels their means were worthy and tries to build on that. Sifo-Dyas is a man was once a Jedi Master that felt betrayed by those that should have supported his efforts to save the galaxy from itself, and now after both sides have been dealt the seeds of their own karma, he has come out of the shadows to --in his eyes--restore order to the galaxy.