I think some monster modles, and special bricks should be added. This way the special bricks could be spawn/respawn points for monsters, and define what monster, and what weapon, so that its easyer. Then make it so the monster's ai is to randomly walk around, Or if you want you could restrict it to roming a certain distance before it stops going further.
I think an rpg side to blockland isn't a bad idea for an add-on.