After a little bit of uncertainty, now it's confirmed: The Book of Boba Fett is a separate standalone series that will tie into The Mandalorian in 2021. Bib Fortuna proved already that characters from Star Wars canon could show up in Boba Fett's tale. Could characters from Legends?
Boba Fett has a rich history in the books and comics of Legends, the umbrella term for those stories that are no longer considered canon by Lucasfilm. That doesn't mean that characters and concepts from Legends can't become canon, as The Mandalorian has proven with the Dark Troopers.
Jodo Kast
Jodo Kast is high on the list of fans who read a lot of the Dark Horse Comics back in the early 90s. This bounty hunter impersonated Boba Fett after it was believed Boba was dead. Kast repainted his armor and traded on the legend of Boba Fett for a long time. When Fett caught up to him, it wasn't pretty. Given that Boba Fett was out of commission for five years or so, it's likely someone could have used his name as Cobb Vanth did his armor, and Jodo Kast would be a good choice.
Cobb Vanth
Since Boba Fett has returned to Tatooine and has an apparent vendetta against those who wronged him (or maybe Bib Fortuna was just in his way), Cobb Vanth seems a likely candidate to appear in the series. Vanth found Fett's salvaged armor aboard a Jawa Sandcrawler and used it in his role as marshal of his remote Tatooine village. He willingly gave the armor to Din Djarin after he helped Vanth defeat a Krayt monster, among the most dangerous creatures in the Star Wars galaxy, but perhaps Boba Fett will have a grudge against the marshal.
Hondo Ohnaka
Another character Boba Fett likely has a grudge against is Hondo Ohnaka. a Clone Wars side character fans fell in love with. A young Boba Fett lost his father's iconic ship, the Slave I, in an effort to take revenge on Mace Windu. Hondo reclaimed the damaged ship and repaired it. At some point, Boba Fett got the ship back but he may be looking to settle some old scores in the new series. Hondo Ohnaka could also be looking for a new gig in the New Republic era.
Qi'ra
It's clear that Boba Fett's new series will deal with the criminal underworld of Star Wars. His radical takeover of what remains of Jabba the Hutt's criminal syndicate could put him into direct conflict with other entities in the galaxy.
One of them could be Crimson Dawn. It's uncertain what became of Qi'ra after Solo: A Star Wars Story, but it's very possible that she is still in charge and perhaps has expanded her influence. Could this lead to conflict with Boba Fett?
Cad Bane
Another fearsome bounty hunter in the galaxy who might be on Boba Fett's hit list is Cade Bane. One of the most powerful bounty hunters in the galaxy, this Clone Wars villain also has a history with Boba Fett. Kind of. One of the unfinished story arcs of the animated series involved Cad Bane being responsible for the dent in Boba Fett's helmet. Some Star Wars books and comics now designated Legends are partially canon, so this arc could be as well.
Dengar
Dengar is another classic bounty hunter from The Empire Strikes Back who made an appearance in Clone Wars. It's likely he could appear in The Book of Boba Fett. In Legends, he crossed paths with Boba Fett numerous times, and they both have animosity toward Han Solo. If Boba Fett does have a list or makes any kind of noise in the galaxy, Han Solo could possibly factor. If so, Dengar could as well (he also cameoed in Jabba's Palace in Return of the Jedi).
IG-88
Another of the six bounty hunters who appeared on the Imperial Super Star Destroyer in The Empire Strikes Back and given his connection to Boba Fett, could possibly return. IG-88 and Boba Fett got into a major battle in Shadows of the Empire, a Legends multimedia event that took place between Empire and Jedi. Boba Fett destroyed the assassin droid in that story, but he remains active at this point in canon. It's possible he could return either as a friend or foe in the series.
Bossk
Bossk is, after Boba Fett, perhaps the most famous of those original bounty hunters. He has featured in Clone Wars in several of the animated series best episodes, including Boba Fett's quest to get revenge on Mace Windu in the second season.
Bossk has been an ally and antagonist to Boba Fett and his potential role in the upcoming series could go either way. In any case, it would be great to see him in live-action again.
If Boba Fett's story takes him on a quest for revenge, it's possible his path could lead to several familiar faces. Boba Fett has claimed to take no sides, but he clearly holds grudges. He may hold on against the heroes of the Rebellion who contributed to his stay in the Sarlaac. Chewbacca didn't have a direct role in Boba Fett's apparent demise on Tatooine, but he is the single biggest impediment standing in the way of Boba Fett's likely primary target, Han Solo.
Han Solo
With the appearance of Luke Skywalker in The Mandalorian, the lid is off on other appearances by major Star Wars characters. The most obvious one in connection to Boba Fett is Han Solo. Solo was simply a job for Boba Fett, but it led to perhaps the worst defeat in the bounty hunter's life. If he harbors any grudge against the former smuggler, it could lead him on a quest for revenge and a potential appearance by the lovable rogue. The only question would be if Han Solo would be CGI like Luke, or Alden Ehrenreich.