The main villain of The Book of Boba Fett hasn't been revealed yet, but fans theorise we could see the return of Qi'ra (played by Game of Thrones's Emilia Clarke). In Solo: A Star Wars Story, she's a high-ranking operative in the criminal syndicate Crimson Dawn, which is led by Darth Maul. Qi'ra becomes Maul's second-in-command after she kills the public face of the syndicate, Dryden Vos. Star Wars comic War of the Bounty Hunters, set between The Empire Strikes Back and Return of the Jedi, reveals Qi'ra rose to the top of Crimson Dawn after Maul's death.
Fans think Qi'ra will return because they caught a glimpse of Crimson Dawn in the trailer for The Book of Boba Fett.