This is open to so much speculation it almost warrants no speculation at all, but since speculation is all we have, let's get right down to it.
I've previously stated that I do believe the first and most popular theory. Since then, things have changed for me a bit. I'm not sure which into which ring my hat will be thrown at this stage, but thought I'd try to put together a post that discusses as much relevant information as is available.
The Most Popular Theory; Snoke is Darth Plagueis
This is a great reddit thread that details some of the key points that lead to this theory. There are many points in that thread that fit, and some that don't. Most of the points on this thread are the ones that led me to initially think this was true.
To be honest, i'm totally unsure at this stage.
What we actually know, what we actually don't, what we actually can't.
This is, to my knowledge, a complete list of points that pertain to Snoke - which allows us to see as clear a picture as can be had at this stage. Some are rebuttals, some are just points based on the facts.
Some answers for common Snoke/Plagueis identity denials
- Some refute Snoke as Plagueis, because Snoke is his name. Darth Sidious had another name, Darth Vader too.
- Some say Plagueis is not canon. Plagueis is referred to in the new Tarkin novel, and in ROTS. Plagueis is a canonical character.
- Some say Snoke is not muun, and Plagueis is muun. That particular point is non-canon. Plagueis could be any species.
- The comic-con refutal. The panel is asked the question "Is there going to be any mention of Darth Plagueis in episode 7, maybe surrounding a certain staff that someone finds"? He is told directly by JJ "no". Some are interpreting this as confirmation that Snoke is not Plagueis. That is not the question the panel is asked.
- The Andy Serkis problem; Andy Serkis says that Snoke has a strange vulnerability. Some suggest that Plagueis would not be vulnerable. If he is Plagueis, he has somehow avoided death. He would definitely be vulnerable.
- Some say there is no way Darth Sidious would not have known that Plagueis still lived. Sidious was able to mask himself from Jedi. He learned this ability from somewhere, and if Plagueis was his master, he learned it from Plagueis.
Some points that might support the Plagueis theory, and might not
All references here are canon.
- The name 'Snoke' does not appear in any canon novel, movie, cartoon, or comic.
- Darth Plagueis real name has never been known.
- There has been a fair amount of discussion showing the similarities in the music when you see Snoke in TFA and when Palpatine/Sidious discusses Plagueis. Some take this as a musical hint.
- Snoke is really old. He tells Kylo Ren that he watched the rise and fall of the Galactic Empire.
- Snoke has had heavy reconstruction surgery, presumably because he's been damaged heavily in the past (ref: TFA novel, TFA), having had facial reconstruction leading to his eyes and nose being misaligned on his face. This supports that the Snoke character has very nearly suffered death.
- Snoke knows that Darth Vader's emotions came into play, and that Darth Vader saved his son. He has intimate knowledge of these events (ref: TFA novel).
- Both Leia and Han know that Snoke turned their son to the dark side.
- The rule of two is canon (yoda refers to it in PM), but Kylo Ren is not a sith. (ref: JJ Abrams empire interview). The 'knights of ren' are affiliated to Snoke, which would imply he isn't Sith either.
- It is never explicitly stated that Plagueis could prevent HIMSELF from death, though he could save others. Non-statement is non-proof, either way.
- Palpatine being Plagueis apprentice is canon (ref: ROTS, Tarkin novel).
- Snoke says he has had apprentices before, but not someone like Kylo, someone who has both light and dark in him (ref: TFA novel) - this flatly refutes any idea that Snoke could be the emperor reborn.
- It is known that when making the film, they toyed with the idea of making Snoke a female, even making a life-size model. They decided against it. That they were willing to entertain it may mean Snoke cannot be Plagueis since Plagueis is definitely a he (ref; ROTS), but then, them deciding against it may be evidence that he is Plagueis - Snoke cannot be a female, because Palpatine said in ROTS it was a male.
Would love to hear your thoughts. Anything I missed?