The Book of Boba Fett may feature Bossk according to LRM Online, therefore take this rumor with a grain of salt, but it also makes a lot of sense for this particular character to show up in a Boba Fett series, so it’s probably not a massive grain of salt that you must take.
Considering the past these two characters share via The Clone Wars, Bossk, as well as many of the famous Star Wars bounty hunters make sense for The Book of Boba, but the Transdoshan in particular has strong ties to the character of Fett. Throughout The Clone Wars Bossk served as a protector of the younger Fett, and seemed quite loyal, so their connection is stronger than just being Hunters. They did have a bond during Boba’s early life, so I could see Bossk being used in a Flashback, or in the current timeline. Either would check out for this series.
The other bit of Book of Boba news is the fact that the series has finished filming, which comes via Ming-Na Wen herself, who plays Fennec Shand. While talking on a 501st Legion livestream, she revealed that principal photography has wrapped, which means the show is now in post, and will surely be ready for its debut this fall. You can check out the stream below. Ming-Na shows up around the 6:56:00 mark.