Why Snoke Should Be Darth Plagueis
When the speculation that Snoke could be Darth Plagueis, I immediately opposed the idea without much thought. The theory grew on me over the months, and by now, I support Snoke being Darth Plagueis 100%. The fact of the matter is that Snoke will either be Darth Plagueis or a completely unrelated and original dark-side entity. The issue with the latter is, obviously, the lack of connection with the core Star Wars storyline. By separating Snoke from the original six films, in my opinion, he instantly becomes a less relevant character that the likes of Darth Vader and the Emperor. With figures like Palpatine, we can see their impact across every Star Wars film, even after their death. If Snoke just begins and ends within the constraints of the sequel trilogy, his impact is significantly decreased. It would be virtually impossible to present Snoke as a villain on par with the Emperor, which they have to do in order to make the plot compelling. If Snoke is less powerful, the audience is going to go, "well, we already beat the #1... how hard can the #2 be?" If he's more powerful, that damages the legacy and achievements of the Emperor, who is the primary antagonist of the first six films. Balancing them as equals is incredibly difficult, but could be accomplished amazingly if Snoke is made Darth Plagueis.
Immediately then, Snoke would be an extremely important character. Secondly, his power and threat would be instantly established. Snoke: the master of the Emperor and the creator of Anakin Skywalker. The fact he taught Palpatine makes the audience know he's obviously an immensely powerful character, but given Palpatine already defeated Snoke, Palpatine's legacy is also solidfed. Assuming Snoke has grown in power since then, the overall verdict on who is more powerful is left ambigious and rightfully so. For the plot, having a villain that essentially created the Skywalker family would be a great way to end the trilogy, and could lead to some epic scenes between him and his descendants: the Skywalkers. Also, Snoke being Darth Plagueis would be a ripe opportunity to make him a tragic character, thus separating him from the absolute evil that is the Emperor.
Hypothetical: Darth Plagueis, on the brisk of death due to the betrayal of Sheev Palpatine, revives himself via midichlorian manipulation, abandons the Sith philosophy due to his defeat and injuries, and then goes out into the Unknown Regions in search of a new understanding of the Force. When Ben Solo becomes active, Snoke finally has his chance to manipulate the Skywalker family, who maybe he even sees as his own family - a chance that was robbed from him due to the Emperor. Whatever the case, Snoke would be tied to the Skywalker family in a way as profoundly as the Emperor was if he is Darth Plagueis, which would be a great thing for the story. Again, making Snoke a generic dark-side villain is not the way to go. All the Star Wars storylines and characters should and must be connected. Darth Plagueis all the way.