The first Boba Fett (Temuera Morrison) was re-introduced in The Mandalorian season 2, which also gave the character a big upgrade. Having previously been divisive over whether he was a great character or just a cool costume, The Mandalorian turned Boba into a much more rounded character with a clearer personality and arc.
All of that will continue in The Book of Boba Fett, which spins off from The Mandalorian, picking up where the season 2's finale's post-credits scene left off: with Boba set to rule Tatooine from Jabba's Palace (or at least, what was Jabba's Palace). But aside from continuing that aspect of Boba's journey alongside Fennec Shand (Ming-Na Wen), it will also be able to shed new light on the Star Wars galaxy, and in turn connect it back to the past while helping pave the way for the future.
As seen in The Book of Boba Fett's first trailer, the show will very much be exploring the underworld of the Star Wars galaxy. Indeed, the footage makes a big point of this, not only in the action it shows but by text literally saying: "Every galaxy has an underworld." Aside from a hint of what to expect from the Disney+ series, it also links back to plans George Lucas had for Star Wars prior to selling Lucasfilm, namely the canceled TV show Star Wars: Underworld and the video game Star Wars: 1313, both of which would have explored different parts of the seedy criminal underbelly in the galaxy and featured Boba Fett as a prominent character.
Star Wars: Underworld would've been set on Coruscant between the prequel and original trilogies, exploring its criminal underworld in great depth and showing a side to Star Wars the movies didn't really dive into. While little was confirmed (though dozens of scripts were written), it was expected that characters such as Boba Fett and Han Solo would make appearances. The former definitely would've returned in Star Wars: 1313, which again would have taken place on Coruscant, with gamers able to play as Boba. Neither project came to fruition, with Underworld deemed too expensive to make at the time, and 1313 scrapped after Disney's takeover of Lucasfilm. In both cases, though, elements of what they were aiming to do - putting a real focus on that kind of grimy side to the galaxy, with Boba Fett a key focus - will be present in The Book of Boba Fett.
This can also loosely pay off some Solo: A Star Wars Story. That included some of those criminal organizations and was seemingly setting up more of that to come with the presence of Maul, ruler of a major criminal empire, and the setup for a story with Jabba the Hutt on Tatooine. The timeline for The Book of Boba Fett is different, but by having him ruling that world from what was once Jabba's seat of power, and dealing with the various factions, gangsters, and crooks within it, then it can deliver on what Lucas and Disney have long wanted for Star Wars.
