While the Emperor was finally defeated for good in Star Wars: The Rise of Skywalker, it's possible that another remnant of the Empire could still return. Following the events of Return of the Jedi and the subsequent Battle of Jakku, the Empire was shattered. While the bulk of the Imperial Fleet headed into the Unknown Regions to be reforged into the First and Final Orders, it's possible another sect headed in different direction entirely.
As confirmed in the sequel trilogy and more additions to the established Star Wars canon, a large portion of the shattered Empire fled into the Unknown Regions in accordance with the Emperor's Contingency, eventually transforming into the First Order led by Palpatine's puppet Supreme Leader Snoke. Additionally, The Mandalorian-era of Star Wars shows set five years after Return of the Jedi confirmed that parts of the Imperial Remnant were still active in the Outer Rim, led by warlords such as Moff Gideon. However, another portion of the Imperial Remnant is potentially still hidden in the region of the galaxy known as the Deep Core.
The Empire Explored The Deep Core As Well As The Unknown Regions
As confirmed in the canonical Build the Millennium Falcon magazine, fact files found in the 36th issue confirm the planet Byss exists, a world located near the Deep Core (a region surrounding the center of the galaxy). In the previous non-canon stories now known as Star Wars Legends, Byss was a key world from which Emperor Palpatine resurrected himself and his Empire as seen in the Dark Empire comic series, essentially the original version of Exegol. While Byss has yet to be used in any capacity beyond this magazine, the details revealed about the planet are quite telling.
Not only does the issue allude to the Rakata Empire from the Old Republic, it's also confirmed that Byss was of great interest to the Emperor. As such, Byss now holds the same significance as other planets important to Palpatine such as Exegol, Jakku, Pillio, and Weyland. However, it's unknown what Palpatine's exact interest in Byss was in the new canon, but it does provide an opportunity for stories after the events of the sequel trilogy and The Rise of Skywalker.
Palpatine Could Have Established Yet Another Imperial Remnant
Just like the Imperial Remnant that fled into the Unknown Regions, it stands to reason that there could also be another Imperial Remnant hidden on Byss and the equally hard-to-navigate Deep Core. If so, it would certainly be a dynamic way for the Empire to have survived yet again, having built up their forces and ready to strike in new projects potentially set after The Rise of Skywalker. Regardless of the means, the potential to honor Star Wars Legends by making Byss a critical world to the Empire's survival would certainly be entertaining.