And, RogueLeader, yes! That sounds very plausible to accomplish (easy for me to say), and fills in a much needed gap. In the theater I quipped to my brother that Mace Windu should have grabbed a second lightsaber and ended it right then. As if that was the mistake.
Yeah, it seems that is what many people said after the movie. Destroying Palpatine should involve more than shooting lightning back at his face. We saw him explode in ROTJ, so something else needs to happen when he explodes this time that makes us think that he won’t come back again. Like some “divine intervention”.
I think the Force ghosts briefly possessing or energizing Rey is a clever way to parallel Palpatine being this Sith collective-consciousness. Plus, tons of the dialogue hints towards this kind of climax. “Be with me”, “A thousand generations live in you now”, “And I… I am all the Jedi.” I think this kind of climax would actually make this final line better, too. And this way, Anakin can participate in destroying Palpatine, which could alleviate any issues people had with his status as Chosen One being potentially contradicted. So, all of this Force ghost stuff we have seen throughout the films, that all started with Qui-Gon’s death in Episode I, all had a purpose that lead up to this moment.
As far as practicality goes, I think a lot of techniques Jonh has used could be used for the brief glimpses of faces we get. I guess beyond that, we could create a mask around Rey that can be used to make a Force ghost glow around her, and then see that energy fly from Rey and consume Palpatine.