When Rey has the dual saber, she has a ring on her left hand that looks a lot like Snoke's. The TLJ visual dictionary said it was made of obsidian with ancient runes engraved in it, & some have speculated that it originated on Mustafar.
A theory making the rounds now, based on current Canon, is that Palpatine learned how to transfer his consciousness, possibly using the ring as a vessel of transference, & that's all Snoke's role was, a temporary vessel.
In the D23 footage, it looks to me like she's standing in the Dagobah cave, so I was thinking 'vision', but the transference theory makes a lot of sense to me.