Your idea is good. In a world where this is non-instanced, you will see many fights occur when one party fights a group of normal mobs, and another party swoops in to take the nearby treasure or NM. Due to that, I feel the only course of action would be to implement this idea as an instanced dungeon. Or at the very least, semi-instanced, where mobs/treasure/NMs are only seen by a certain party, similar to how you can't see another person's leve mobs until they are claimed.