Star Wars continues to set up the return of Grand Admiral Thrawn, and it's now evident that the villain will soon become the galaxy's next phantom menace. Just like Palpatine, who heralded the return of the Sith as Episode I's titular "phantom menace," Thrawn has become the new threat working in the shadows in the years following the original trilogy. To that end, there are quite a few similarities between Emperor Palpatine and Thrawn within this context, especially with the Grand Admiral having yet to fully reveal himself to the galaxy at large during the New Republic era.
Following his recurring role in the animated Star Wars Rebels series set just before the events of A New Hope, Thrawn seemingly vanished from the Star Wars galaxy. However, it's since been revealed in The Mandalorian that the Chiss Imperial officer is still alive, no doubt preparing and planning his next moves potentially in the hopes of an Imperial resurgence. Regardless, Thrawn can be easily identified as the next phantom menace based on what's currently known, as well as what's actively being set up for the dynamic villain in the galaxy far, far away.
Thrawn Disappeared From The Galaxy (Just Like The Sith) 
Although Thrawn's strategic mind would have been invaluable to the Empire as they sought to root out Rebel cells prior to their assembly as the Alliance, Jedi Ezra Bridger removed the Imperial officer (and himself) from play in Star Wars Rebels' series finale. Using his bond with a pod of space-whales known as purrgil, Ezra influenced the hyperspace-traveling creatures to pull Thrawn's Star Destroyer into the Unknown Regions, with the Grand Admiral and Jedi both onboard. As such, neither of them was active during the events of the original trilogy. However, Thrawn was confirmed to still be alive by Ahsoka in The Mandalorian season 2 ( and presumably Ezra, as well).
