I like the idea but... maybe having the bounty box be the access key is not a solution (you lose it when you die or when you kill a baddy of the desired type). If the BK people were crafty (and they are) they would figure out that they could grab the bounty box and then head through to the area and fight em after dropping the box...
Unlikely this will happen because there are just so many difficulties that arise, but maybe added areas for bandit, crab, and rats could be implemented... Those are the ones I feel the pinch with, in any event. Something like extra baddy fighting locations on Oasis island, perhaps?