Snoke's thoughts about Ben.
Ideal embodiment of the Force, both Light and the Dark Side.
Why would a villain like Snoke be interested in the Light Side? If he is in fact Sidious, or Plagueis, he wouldn't have interest in it. Not that much.
I agree that a completely new character would be lame, but what if he actually isn't any of the previous Sith but actually a Jedi padawan from Star Wars: Rebels? Or he actually has some connection to him. xD Ezra Bridger.
Ezra is a character introduced in the Star Wars Rebels series. He is a young Jedi Padawan of Kanan, a Jedi who managed to survive Order 66 and is now a Rebellion fighter. He is very gifted with the Force.
I'll star with obvious. In concept art, Snoke has blue eyes, same as Ezra.
Ezra obsession is collecting helmets. He has faced Darth Vader and admires him so him getting his helemt (We didn't see Luke carrying it in return of the Jedi) isn't really so hard to believe. Ben had to get that mask from somewhere.
Ezra did used the Dark Side couple of times prior to the season 2.
What got me thinking that Snoke or Knights of Ren have some connection to Ezra is a mid season 2 trailer.
We already talked about Gray as the concept of the Force.
They are not canon now. They exist only in the Legends department. But, the line used to describe Snoke's thought about Ben - He is perfect embodiment of both sides. Sounds like a Gray concept to me.
In the trailer we see Kanan in front of a man, who uses both Force and is skilled in lightsaber combat. And he is all in gray. Plus, look at the logo above Kanan. It looks really a lot like a logo used in EU for Gray Jedi Order.
Trailer also gives us strings about Ezra's connection to the Dark Side. In video below Yoda speaks about how his abilities are growing and with them the danger, too. Next scene we saw Vader approaching, and Ezra admiring Anakin in video that Ashoka shows him. Back to my prior assumtion, I think this is the reason why Ezra would track down Vader's helmet.
In the next shot we see a Gray Jedi talking about Ezra, how the Dark Side calls to him, how Ezra used Sith Holocron (Not really a Jedish thing), how Darth Maul returns and is actually calling himself Ezra's Master. He says Ezra seeks knowelgde.
Ezra already talked to Yoda. He has tuteleage of Kanan and Ashoka Tano. Why would he seeked anything from a Sith? He is rebelious nature as seen here.
So him wanting to get knoweldge on both sides, appearing of the Gray Jedis, and later in Force Awakens appearing of Supreme Leader Snoke who believes in both sides is just too much to be a coincidence. Plus Kanan was in fight with this Gray dudes.
Kanan is holding his own lightsaber but also a red one which is always a symbol for Sith. Maybe they are fighting to show him that both sides are needed to have peace.
Ben lightsaber - We know his saber is unstable and we also know this:
This lightsaber was first used in Great Scourge of Malachor. Scourge - A means of inflicting severe suffering, vengeance... So Sith being connected with it, doesn't seem odd.
Ashoka, Kanan and Erza come to a very Sithy place.
Erza finds this:
This crossguard is actually the first one seen in canon Universe, as Star Wars Rebels timeline is before The Force Awakens. So Ezra is the only canon character that actually hold a stable saber. Someone had to teach Ben how to make one, and it was most likely Snoke who did it.
Not to mention that Erza is a rebel who had ties with Leia. She witnessed the plans of Death Star. How in the hell would Snoke know how to built the devastatin weapon similar to Death Star if he hadn't have plans for it. The Death Star was destroyed twice, so most probably the only plans left are the ones with the Rebel alliance.
Seems far fetched for Erza to be Snoke, but his connection to the concept of both light and dark side (same as Snoke), him admiring Anakin and having obsession with helmets (Kylo couldn't possibly get that helmet on his own, nor we saw Luke carrying it), his training by Kanan, Ashoka, Yoda, Maul implies that he will have a vaste knoweldge of the Force, him wielding the first crossguard saber we saw in canon, him using both sides, defying the logic of Light and Jedi, his lighsaber skills who are high enough to match Vader (Ben did not create the Ren Knights, somebody did it before him. As there are not really so many great swordsman in the Galaxy except Erza/Obi Wan/Ashoka) implies in some manner that Erza will actually have some connection to Snoke and the First Order.