You could have mobs camping together in groups of 2-4 with wandering groups of another 1-3 mobs with nearby stationary groups of another 2-4 mobs. Perhaps 1 of the mobs in the group are SLIGHTLY stronger then the others.
This allows a rogue the chance to be strategic and pick a smaller group to attack and use his first sneak attack skill to quickly take out the stronger mob, while also needing to keep an eye out for wandering mobs who might jump into the fight.
An AOE focused player can choose to pull multiple group since the groups would be close enough together.
This would also pose a fun challenge to a player that kites, especially a single target kiter, you would need to watch how close you get to the other groups and also watch out for the wandering groups so you don’t pull an overwhelming number of mobs you cant effectively heard together.
Slightly off topic, but this is also a fun mechanic to use in boss fights, having a boss cast a spell that spawns adds can be a fun way to promote group play, single target dpsers can focus down the main boss while aoe dpsers burn down the adds