Taking bounty hunter doesnt make any sense (in singleplayer at least).
In the marine bases are bounty boards. These boards could have bounties of some random generated npcs with different bounties and random skins. If you take a poster, it becomes a map to this npc, so you can claim your prize, if you are strong enough. Depending on your doriki the bounties available are higher or lower and if possible have a random devil fruit and haki.
And this is is all customiziable in the config: you can activate/deactivate it, change what the highest/lowest bounty can be, if they can have haki/devil fruit.
Thats all thank you! ^^