Barret's main mission while joining the Bright Bats is to strengthen his comrades and became the number 1 gang on the island.
This could be used as a good main quest, or sub-quest:
There are multiple territories divided by the other gangs, if Barret enters to enemy territory they will just kick him off, or even attack him.
The main mission would be getting in the bike and do some vandalism, crushing trashcans, people (in one prototype video it's shown that is possible to ram the bike against people) damaging police vehicles, every vandalism that can be done aboard the hovering bike. This will cause the police to chase Barret, and eventually they will go up on their blacklist, other bikers will try to stop him too. While defeating them you open new territory to explore, finding some treasure chests and such.
It seems the biker gangs would be a recurrent or even main theme in the prototype version, there can be many possibilities to play around with this quest, thinking many ways to make the police note them, how many objects can be destructed, squashed, stolen, all without loosing the fun and charm of the legends series.