As the teaser trailer for Star Wars: The Rise of Skywalker cut from the ruins of the Death Star II to black, watchers heard Emperor Palpatine’s unsettling cackling, indicating he'll play some role in the upcoming film despite his apparent death in Return of the Jedi. According to director J.J. Abrams, the upcoming film will pit the sequel trilogy's heroes "against this unspeakable evil.” In a Galaxy wrecked by its third Civil War in four decades, it’s difficult to pinpoint what the Emperor even wants anymore. However, there might be some foreshadowing in previous Star Wars films and books that gives us some idea of his goal when he returns.
Over the course of the prequel trilogy, Palpatine’s plotting, actions and motives seemed straightforward enough. Step One: Destabilize the precarious peace of the Republic through commercial blockades, separatist movements and forced Jedi Council coups. Step Two: Grudgingly accept Chancellorship. Step Three: Obliterate the opposition and grab all political and military power.
His plan works perfectly, and he even manages to corrupt Anakin Skywalker, the Chosen One, in the process. This leaves him pursuing his three passion projects around the time of the original trilogy: collecting ancient Sith and Jedi artifacts, sending explorers to probe the Unknown Regions and building Skyhook, a gorgeous sub-orbital private garden. He also uses some of this time to train and use Darth Vader, with comics set between Revenge of the Sith and A New Hope having Palpatine send his apprentice to pursue his nefarious ends.
It becomes kyber-crystal clear in Chuck Wendig’s Aftermath trilogy Palpatine planned for the Empire to die with him. He didn't spend much time directly ruling, but he went out of his way to plan the Contingency, a back-up protocol based on the certainty that the Empire should not survive him, and should that happen, his loyalists would ensure the Empire’s destruction and its retreat into the Unknown Regions to regroup for some nefarious purpose.
So, if Palpatine wasn’t pursuing historical glory or a lasting legacy, what was he after when he became Emperor? Eternal life plays a role, especially since he was the apprentice of Darth Plagueis. Immortality pops up enough in the Sith tradition as to not be that special; actually, “essence transfer” is so common as to be part of a Sith Lord’s toolkit since the times of the Sith Empire. However, the ability has pretty major drawbacks. What self-respecting Dark Lord would want to spend eternity trapped in a mask, like Darth Momin, or in a pendant, like Karness Murr, or in a mummy, like Darth Tenebrous?
What is undeniable is that Palpatine managed to somehow, in some way, survive Darth Vader’s throw and the collapse of the Death Star II, so whichever plans he’d had before his demise are going to take a backseat to returning. This means that if he’s immortal, by way of classic Essence Transfer into an inanimate object, he will be on the look-out for a living vessel, preferably Force-compatible, to get back on his feet. That body could be either Kylo’s or Rey’s, but there are other six Dark Side users, the Knights of Ren, that have potentially spent the last two movies on an ancient artifact treasure hunt, so it’s more probable that the first sentient character Palpatine possesses is one of the Knights, and that he does it through one of the mysterious artifacts that he collected during his time as the Emperor.
Taking someone's consciousness over looks a lot more like “unspeakably evil,” and there's a chilling precedent in the Star Wars Expanded Universe, particularly in The Knights of the Old Republic: Emperor of the Sith Empire. Lord Vitiate, who used a similar technique, managed to live and rule for 1,300 years. Vitiate kicked off his power trip by performing the Ritual of Nathema, which sucked the life out of the planet of Nathema, stripping the Force off the very fabric of reality. Vitiate planned to eventually do this to the entire Galaxy. This act would definitely qualify as a proper act of unspeakable evil worthy of the final episode of a legendary galactic saga.
Besides, the Galaxy, as The Rise of Skywalker begins, is in the perfect state of disarray for a Palpatine's ghost to sweep the Force home. From the First Order, internally divided, to the fragmented and fragile Resistance, to the way Kylo Ren and Rey seem to have dug their heels into each opposing side of the Force, it looks like the perfect storm for a Machiavellian divider and conqueror Sith Emperor.
Palpatine is like a lot of different villainous entities in the series, like the Sarlacc under Jabba’s barge, patiently waiting for the bodies to drop in; the Exogorth that almost ate the Millennium Falcon; the garbage disposal monster, slithering slimy tentacles around our heroes; or Starkiller base sucking suns dry to destroy planets with phantom energy. The constant imagery of insatiable devourers in the movies fits Palpatine's motives perfectly. Even his out-of-character Skyhook gardens bring to mind a cunning predator, lying in wait to hook a Skywalker and bring the universe crashing down with a timely tug, all for the sake of becoming the most singularly powerful being in the universe.