I hope they go mysterious with Palpatine's return. Don't just have him be his physical old ROTJ self after getting shafted and exploded by Vader. He should not even be there physically in any form.
I can see a scene where Kylo finds the DS2 throne room, walks up to the throne, sits down, confident in himself finally having reached the end of his Galaxy spanning Empire nostalgia tour and owning it all now.
Then a slow cackle from all around him starts to grow, stronger and stronger. (think Jabba laugh when Leia thinks she has just freed Solo)
Kylo starts to show fear on his face and something is taking him over - until Palpatine basically completes an Invasion of the Body Snatchers on Kylo and is now him.
He basically groomed and lured the most powerful force using idiot towards his resting place where he needed his body to do a Voldemort and live again.
Jedi ghosts inspire those that can see them.
Sith ghosts should be ones that possess others out of selfishness.