Star Wars set up Palpatine's new Force Drain power all the way back in 2015. It's now becoming clear that Lucasfilm didn't really have much of a plan for the sequel trilogy. Although they initially attempted to pretend they'd planned Palpatine's return all along, Colin Trevorrow spoiled that when he credited J.J. Abrams with the idea. According to Trevorrow, the Emperor most definitely wasn't part of his script for Star Wars: The Rise of Skywalker.
Curiously, though, Chuck Wendig's Aftermath trilogy does effectively foreshadow a lot of Star Wars: The Rise of Skywalker. It introduced a Sith cultist named Yupe Tashu, who was absolutely convinced that Palpatine would return from the dead, that he would be somehow be "enfleshed" again. In fact, on close examination, the very first book in the trilogy established a new Force power - one that the Emperor went on to use in Star Wars: The Rise of Skywalker.
The third act saw Rey and Ben Solo stand united against the Emperor. Far from threatened, he was delighted, because he recognized that he stood in the presence of a Force Dyad. The Emperor abandoned his plans of switching bodies, and instead used a power called "Force Drain" to absorb the power of their bond into himself, rejuvenating himself with the power of the Force that he had stolen from them. The ability had been in the old Expanded Universe, but surprisingly it was absorbed into the new canon courtesy of dialog from Yupe Tashu. In one scene, torturing a captive Wedge Antilles, Tashu rhapsodized about the power of the Sith. "Did you know that the Sith Lords could sometimes drain the Force energy from their captives," he told a prisoner. "Siphoning life from them and using it to strengthen their connection to the dark side? Extending their own lives, as well, so that they could live for centuries beyond their intended expiration?"
This is a perfect account of the power Palpatine would use when he realized he stood in the presence of a Force Dyad. Of course, there's a certain irony to the fact that he didn't use Force Drain on captives; rather, the Emperor was powerful enough to steal the Force from people who effectively had a lightsaber at his throat. The stolen power of the Force bond - something so strong that it had frightened even Jedi Master Luke Skywalker - was enough to repair the Emperor's broken body, and allowed him to summon a Force Storm unlike anything seen in previous Star Wars movies.
It's worth noting that Abrams was still involved with Star Wars back when Wendig was writing his books. That raises the possibility that the connections between the Aftermath trilogy and Star Wars: The Rise of Skywalker aren't coincidental; perhaps this trilogy in fact inspired Abrams to bring back Palpatine, and to give him the powers he did.