So I have one crazy idea for how one could theoretically give Snoke an origin in the Last Jedi, and this would be using the Darth Plagieus theory from before the film came out. This could probably be a bit more refined, but I just thought this up now.

Basically, in the mirror vision scene Rey has, she never asks about her parents, she just walks toward the mirror, reaching out to it. As she does, we would begin hearing the line from ROTS where Palpatine talks about Darth Plagieus being whispered in the background, with music building it up. Eventually, Palpatine would say Darth Plagiues the Wise, which would start repeating a bit, getting louder as it does, and we’d then get a snapshot of Snoke’s face in the mirror, and that is when Rey wakes up from the vision.