I would prefer Boba as a villain with a bit more screen time than as the focus of an entire movie.
HOWEVER, if I were to do it, it would be an intergalactic remake of The Good, The Bad, and The Ugly replete with a 3 way duel in the end. I would do Dengar, Bossk, and Boba bringing each other in for bounties. It could be fun.
I would love to see a Vader movie focusing on the period between ROTS and ANH. The scenes with Vader in R1 were the best in the movie.