What would you say if I suggested that Supreme Leader Snoke, Darth Plagueis, Darth Sidious, and Darth Vader are all the same guy? It's pretty farfetched and outright unlikely, but putting that aside for a moment, Reddit user HeronSun has constructed a complex and intelligent theory which could explain the origin of Supreme Leader Snoke. Admittedly the theory is pretty unlikely, but let's just enjoy the ride.
HeronSun begins by explaining a little about how Sith lords are named, which is through a central characteristic. Darth Sidious's name stems from the word 'insidious': to harm slowly over time; 'Vader' comes from the Dutch word for "father"; Darth Tyranus is the tyrant of the Separatist movement... you get the point. So, HeronSun says, perhaps Plagueis's name stems from being like a plague.
The theory is as follows: what if when we meet Palpatine in the prequel trilogy he is in fact Darth Plagueis, his own former master who he claimed was slain. There's been speculation about this fact amongst the fans, who note that the Emperor speaks fondly of Plagueis when discussing his passing, describing it as a "tragedy." So what if Plagueis, who mastered the ability of keeping alive his loved ones, learned to survive through inhabiting the body of his apprentice?
After Palpatine's battle with Windu he didn't seem to care that his body was "scarred and deformed," because he had been grooming Anakin as the perfect vessel after encouraging Anakin's midi-chlorian birth decades before.
The problem is that Plagueis must wait for his living body to die before he can move on to someone else, hence why he raised a series of powerfully Force sensitive apprentices and occasionally encouraged them to kill one another. Essentially, what if Plagueis continues to survive in a parasitical form?
HeronSun then explains:
What Plagueis did not predict was Anakin's body succumbing to severe injury, but remaining alive. Of course, Plagueis would want his new body to have limbs that function, so he created the Vader suit in order to keep Anakin alive and fully functional.
Not wanting the ruined body of Lord Vader, Plagueis turned his attention to Luke, who refused to become his apprentice, an infuriating turn of events which led to Plagueis's downfall. The consequence being his own death and the death of his apprentice, who's ruined body he then inhabited.
No longer obsessed with acquiring the perfect body — at least for now — Plagueis inhabits the ruined body of Vader and utilizes cloning technology to replace his mechanical arm.
Linking the theory in with traditional Star Wars lore, HeronSun then states:
After hearing about how the Jedi describe the Force and the Dark Side, which can be akin to a cancer, a sickness... or a Plague, one can safely say that Plagueis is the manifestation of the Dark Side of the force. If that's the case, then Plagueis must be destroyed to bring Balance to the Force.
Finally, HeronSun theorizes that Supreme Leader Snoke is in fact the ruined body of Darth Vader, inhabited by Lord Plagueis who had spent decades as Lord Sidious. This could of course explain their similar scars.
The theory finishes with the suggestion that the youngest of the Skywalker descendants, Ben Solo, would then be responsible for bringing balance to the Force.
You can find the entire theory here.