Given that Rogue One is about putting together a team to steal the Death Star plans, and Solo is about putting together a team to do the Kessel Run, I can put money on the Boba Fett movie being about putting together a team to go after a big bounty.
Maul obviously. Then Vader will come in at the end and we'll have the long awaited Vader vs. Maul fight
They already did that in Legends so I imagine a movie would have a bit more to it. But just because it's part of Star Wars doesn't mean it has to be all lasers and space combat. I'd quite like to see Disney try and broaden the pool rather like they've allowed Marvel to do with some of those movies, so a decent character-driven piece with Ewan McGregor could be good.
And for those who haven't read that book, do yourself a favor and pick it up. It is fantastic.
On the other hand if you stick to the more familiar/iconic armour-wearing Fett at the other end of the timeline you've then got a hell of a lot of heavy lifting to set the scene at the start so everyone understands where we are and how we got there (whether or not they use a get-out-of-jail-free text crawl) and your hero is in full-body outfit. Alright that worked for Marvel with Iron Man and Deadpool, but both Tony Stark and Wade start out without the suit and are much more jocular and engaging characters (plus fourth-wall-breaking in Deadpool) than the monosyllabic Fett we've seen previously.
Let's not forget how divisive a character Fett is as well. You have people that love him and those who hate him, with very little in between. IMO an interesting take would be Boba post-Sarlacc. Make it so before the incident he really was just an average bounty hunter, but he was able to live off his father's reputation. Now that he was so ignominiously defeated he has lost everything, he is the laughingstock of the bounty hunting community. Time to step up his game and earn a reputation as the galaxy's greatest bounty hunter.