Before the Brig was removed from the Sloop, one idea that I had which I felt could be seen as potentially balanced and adding a buff to defending against boarders was as such:
If you kill a Pirate on your Ship, rather than the normal Revive state before the Ferry they would be shifted to the Brig instead. They would stay in the Brig only as long as the normal Ferry respawn delay before being safe teleported back to their own Ship. This is only a pseudo buff to boarding prevention since Revive was not an option when the game launched so fending off a boarding party as a smaller crew was more manageable than it can be these days (where unless you wipe them there is a good chance they maintain their numbers via revives unless you can maintain really good pressure).
It would make the play of boarding a little more dangerous and shake up the meta some in that regard (while not making it completely impossible or pointless to try), and would have added some general "use" to the Brig (in particular on a Sloop where you couldn't Vote people to the Brig generally). Crews could potentially attempt to Parlay in this time and negotiate with their attacker (though again nothing is forced). It would scratch a little of that RP itch for those looking for it (capturing prisoners and not just locking up your own crew).
Now that the Brig is no longer on the Sloop though this just doesn't really work anymore (unless they bring it back and relocate the bed - which still needs to be raised up some with all the Rats running around now too). I think that could have been a fun and generally balanced mechanic.
C'est la vie.