Emperor Palpatine returned in Star Wars: The Rise of Skywalker, but why did he wait until after Supreme Leader Snoke's death to do so? Despite having been killed in Return of the Jedi, somehow Palpatine returned in The Rise of Skywalker, with it later confirmed that the version of the Emperor in J.J. Abrams' movie was a clone. Even with that, though, there are still plenty of questions to be asked about his plan.
Also a clone was Snoke, as confirmed in The Rise of Skywalker. Snoke was a creation of Palpatine's, who had a vat full of copies of the Supreme Leader. Snoke, then, was a puppet used by Palpatine, albeit one with some independence and free will (as a strandcast, an artificial genetic construct), who was struck down by his apprentice, Kylo Ren, in Star Wars: The Last Jedi. It's into that power vacuum Palpatine finally makes his comeback, but The Rise of Skywalker doesn't explain why he waited so long.
Snoke was important for Palpatine in re-gaining and exerting control in the galaxy via the First Order, but even more so for influencing Kylo Ren. Star Wars canon has shown that Palpatine was taking an interest in Ben Solo since before he was even born, and that continued throughout his life. In this case, Snoke was simply a surrogate; since Palpatine couldn't train him directly, he used the Supreme Leader as his proxy to do that through. In this sense, Snoke was also serving as a placeholder until Kylo Ren proved himself ready to take over, which is when Palpatine could fully enact his plan to return. That's what Kylo Ren achieved with killing Snoke in The Last Jedi, showing he was ready.
This was confirmed by the novelization of The Rise of Skywalker, which revealed that Palpatine had designed Snoke as a test for Kylo Ren, saying: "You did well to destroy him." Although Snoke and Kylo Ren weren't Sith, they still allowed Palpatine to use the familiar Master-Apprentice dynamic in the same (or at least similar) way; with the latter killing the former, then Kylo showed he was worthy of ascending to become the leader of Palpatine's planned new Sith Empire, and to take over as the Emperor as the new vessel for all of the Sith.
Palpatine was unable to leave Exegol because his clone body was failing him, and so he had to wait for the right timing to strike. When Snoke was alive, things were progressing reasonably well enough: it wasn't possible for him to return just yet, but the First Order was maintaining some control, even with the Resistance's best efforts. But with Kylo killing Snoke and his heir finally confirmed, Palpatine could safely (or so he thought) announce his return to the galaxy and truly put his Final Order plan into motion, as seen in The Rise of Skywalker.