Bringing Palpatine back in Rise of Skywalker was a really good idea. He was the true big bad of the original series and we were given the story of his rise to power in the prequels, so it makes sense for him to haunt the rebellion from beyond the grave. Making it tied to his Sith powers was also a great touch. The problem with it though is that it comes out of nowhere. The first hint that he was involved was given in a teaser trailer for the very movie he was being introduced in. It was a well executed hint, but it should have happened in the actual movies, particularly TLJ.
They could’ve started teasing it as early as TFA by telling John Williams that Snoke was a construct/tool of Palpatine. The man is a genius when it comes to sound design and character vignettes. He could’ve found a subtle way to put Palpatine’s theme into Snoke’s theme that would give keen-eared viewers an early hint as to where Snoke came from. It would’ve made for a very subtle and brilliant introduction to the concept. But, they wouldn’t want to do much more than that in TFA. Revealing Palpatine is alive in the first act of the trilogy takes away a lot of the impact. Palpatine has been the true villain of the past 6 films and his death in RTJ was a grand culmination of his character arc. Bringing him back is a master stroke, but bringing him back too early undercuts the impact of RTJ.
However, a full reveal in TLJ is the perfect timing and would be akin to Vader’s “No, I am your father” without being a complete rip-off of the original or a janky subversion like we got with Rey’s parents. The first hint in TLJ would’ve worked well during Snoke’s death. Imagine if Snoke had yelled out when he was killed, rather than staying silent. But overlayed with Snoke’s voice could be Palpatine’s. Subtly and easily lost if not listening for it, but present enough to prick at the minds of the super fans. For the full reveal though, that would need to happen at the end, when Kylo Ren finds the dice. Rather than having him kneel and bow his head as Rey closes the door on him, have him rage as he has done time and time again throughout the films. Then, as he rages, let the laugh begin and rise in volume as well as raucousness as Kylo continues to rage before cutting back to the entire rebellion on the Falcon. You drop that laugh at that moment and the audience would lose their collective shit. The true villain of the series possibly returned from the grave and happy about Kylo’s anger? Cinematic gold.
Plus, it dovetails into Rise of Skywalker far better than what we’ve got now Palpatine’s appearance as it stands is interesting, but confusing and ill-conceived. There’s no reason for him to be there and while the explanation given works, it lacks gravitas. A reveal like Palpatine needs to be telegraphed in the story somehow to avoid feeling like the Hand of God stepping in to provide plot.