One of the great casualties of Disney's purchase of Star Wars was the Legends continuity. While Splinter of the Mind's Eye was the first entry in the Star Wars expanded canon, its popularity exploded thanks to Timothy Zahn's Thrawn Trilogy -- Heir to the Empire, Dark Force Rising and The Last Command. These books expanded beyond Return of the Jedi, showcasing the New Republic's development, outlining how remnants of the Empire still clung to life and how the leadership of one central villain -- Grand Admiral Thrawn -- almost eradicated the galaxy's government in its infancy.
However, as it turns out, the Thrawn Trilogy has a lot in common with the shows being developed by Dave Filoni and John Favreau. Star Wars: The Mandalorian and its spin-offs, Rangers of the New Republic and Ahsoka, follow tackle the same general concept as Zahn's Thrawn Trilogy, seeing the New Republic struggle as remnants of the Empire attempt to take back control.
THRAWN'S ROLE IN LEGENDS IS SIMILAR TO THE MANDALORIAN
The Thrawn Trilogy explored a few core concepts that The Mandalorian also seems intent on tackling. The books dove into the aftermath of the Empire, both in regards to how the New Republic blossomed in the wake of war and how the Empire clung to life despite being leaderless. It also touched on the criminal underworld's complicated relationship with the Empire and the New Republic, as well as learned about key members in the Imperial Navy that proved instrumental in Palpatine's reign. In short, fans saw what the Galaxy was like, ever-so-briefly, without any shade of Palpatine over the proceedings.
In essence, this is also the world that Star Wars: The Mandalorian explores. The whole story is centered on the experiences of those living on the outskirts of the New Republic, all while the Empire tries to take further root. While so far the majority of the threats have centered around Grand Moff Gideon, Ahsoka confirmed that Thrawn is still an antagonist in this era of the story during her appearance in The Mandalorian.
TRANSITION IN STAR WARS
The Thrawn Trilogy served as a transition from the Original Trilogy to Legends. Without Thrawn, there would be no Dark Empire, no Yuuzhan Vong War and no Second Galactic Civil War. In this sense, the books worked both as a sequel to the original films, but also as a bridge to later stories that would be told.
Likewise, the expansive story of both Star Wars: The Mandalorian and its spin-offs serves as a transition to the Sequel Trilogy. Fans are seeing the groundwork laid that can build up the New Republic and the Empire post-Jakku, all so stories can be told in later eras of the saga. It seems very likely that future Star Wars projects will build upon the post-Mandalorian and pre-The Force Awakens era to explore plot points set up in these series, much like how Rogue One, Solo and Star Wars Rebels all explored the middle years between Revenge of the Sith and A New Hope.
THRAWN AS THE CENTRAL VILLAIN
Thrawn was brought back into Star Wars canon thanks to Filoni's Star Wars Rebels, which established him as a threat to the Rebellion around the period of A New Hope. Legends, similarly, explored Thrawn's life before his trilogy of books, outlining why he was a threat but not the primary threat of the galaxy.
However, in the present canon, without Darth Vader or Palpatine playing a primary role in the events of the galaxy, Thrawn has a chance to play the role of Big Bad. Sure, the Emperor remains a constant threat in Exegol, reviving himself with the Sith Eternal, but with him taking an indirect role currently in the canon, Thrawn can rise to power.
Part of the reason for Star Wars' rise in popularity in the years after Return of the Jedi was due to Thrawn, whose strategy and cunning felt distinct from prior villains in the franchise. Now, The Mandalorian can offer Thrawn another chance to prove why he's one of the most dangerous men in the galaxy far, far away.