This idea might turn some people off, but presuming they actually do go this route, this is how I would approach the spin-off from a totally unconventional, non-starwarsian, approach...

It could be very cool to see two different stories that jump back and forth between one another:

-The protagonist/anti-hero would be a mysterious drifter.
-The main story would follow the drifter, who has taken up the mantle of Boba Fett, after killing him 15 years ago. He is on the hunt for a target with a bounty on their head. Drifter Fett is killed at the end after a harrowing fight (as a plot point, he'll achieve some moral goal along the way, so his effort wasn't in vein).
-The secondary story takes place 15 years earlier, as the drifter is being hunted by the original clone Fett (who has a galaxy wide reputation) and he ends up killing him and taking the mantle.

Obviously there would be more to it than that, but the two stories would contrast the rise and fall of the drifter as Boba Fett. The main storyline would start and end the film, while the secondary story would play out intermittently throughout, almost as flashbacks. It's completely out of the Star Wars storytelling style, but I think they need to mix it up for the spin off movies anyways. And naturally the final scene of the movie, would be the man with the bounty on his head, picking up the Boba Fett helmet.

To me, this would establish Boba Fett as more of a legend than a man. And that given the rifht circumstances, the Boba Fett mantle can be carried on.