I don't like the idea of Palpatine returning (even as just a force ghost), not because of plausibility or canon issues, but because it's yet another example of this trilogy leaning on the original instead of, well, being original. It's not just that it's another returning character, but also that, if he becomes the main villain (either by being reincarnated, possessing Kylo or just posing a threat to both sides), chances seem good that Kylo will eventually turn on him and redeem himself, exactly like his grandfather did. The trilogy that started by ripping off ANH may well end by ripping off RotJ.
On top of that, it highlights one of the main problems with Kylo: that's he's not a good enough villain on his own. Instead of trying to fix that in the third movie, the writers are seemingly throwing their hands up and bringing back the most notorious villain of the franchise to patch the problem. Admittedly, I'm not sure how they could fix Kylo at this point, but it's hard for me to like their alternative solution to escaping from a corner that they shouldn't have written themselves into in the first place.