I’ve been watching everything with subtitles on, so my mind is trained to reading a little right now and I couldn’t make out a few lines Palpatine said, but I took it as him just harnessing their life forces and taking in their spirit and power. I wasn’t thinking inside of Palpatine was the spirit of Plagueis or anything, just that they were probably implying Palpy stole his power, and that was why Sidious was the biggest baddie of the Sith. It at least ties a bit into Episode III when he keeps talking big game about cheating death, but his whole plan was just to eventually body jump into Anakin until it was ruined when he had to wear a suit to live.
I thought it would have been funny if they had gone through with Rey finishing the ritual and then Daisy Ridley would have had to act like Palpatine until Ben pulls his soul out of her or something.