In Star Wars: Return of Jedi we see Emperor Palpatine killed by Darth Vader when he could not bear Luke being tortured by the Emperor. In Star Wars: The Force Awakens, we were introduced the Supreme Leader Snoke who controls the First Order. We never got to know any back story of him. Snoke remains a mystery. There is a theory that is speculating in 4Chan of Snoke’s origin.
Palpatine is back but not how you’d expect. A copy of his conscious is held within an item (the MacGuffin).
He possed a body and rules as Snoke. When Snoke was cut in half, the conscious returned to the MacGuffin.
Kylo needs to destroy MacGuffin to stop Palpatine reclaiming First Order through another host. KOR returns to Kylo with info on where they think it is. Kylo and Rey team up to destroy Palpatine once and for all. Twist: a KOR (Matt Smith) is already possessed when they return to Kylo and eventually turns on him.
Lucasfilm has been leaving bread crumbs and have already made this possible in the canon (Momin mask). Palpatine has been the main villain since Episode 1.
With Palpatine returning in Star Wars: The Rise of Skywalker, this theory makes sense. Kylo killed Snoke to take control of First Order. He does not want Palpatine controlling it again and seeking revenge on him.