I remember playing some very generic asian-market game a few years ago. Very generic but it did have one thing that was interesting! If a lot of a particular kind of mob were killed within some period of time it would cause a higher level kind of mini-boss to pop that would I belief specifically (from a larger agro radius) attack people who were attacking it's kind. Kill rats and a bigger super-rat would pop. It would drop better stuff then the generic mobs of course. I always thought that was a interesting feature.