Quoted from TheLaw:
Didn't we already have a movie or 2 with like 4 billion Boba Fetts? Wasn't that interesting... Whole point of the character is he's an unknown.
They can do a lot with a Boba Fett movie. Consensus #1 fan favorite character, next to Darth Vader. There's a lot of mystery surrounding his backstory. The Mandolorians have never been presented on film before. Lucas may have told an origin of Boba Fett in the prequels, and that he was a clone of Jango Fett. Most fans weren't happy with that explanation, and it could be easily fixed.
I've often heard a different origin story, where an unknown ruthless bounty hunter looking to make a name for himself killed the clone Boba Fett, stole his armor & took his name for himself. Cast that Jango Fett actor again & kill him. Fans would get a kick out of that. That would also give the writers the freedom to do whatever they wanted with the character. They could tell that origin story.
Or a better idea might be to tell the story for how Boba Fett delivered Han Solo in carbonite to Jabba the Hutt. Once word spread that Boba had his hands on the biggest bounty in the galaxy, every other bounty hunter (like Bossk, Dengar, etc) tried to hunt Boba down & claim Solo for themselves. That could be a pretty crazy adventure & would provide the fans with something they're really wanted to see.