Here is my idea, but adapt it in any way if you like it.
When a chaos warrior is engaging a hero that is alone, the chaos spawn keep a varying distance from the player based on their weapon reach, and try to surround him as he fights their boss. They’ll cheer if he hits the player.
If the player wins, the chaos warriors will rush in to try to kill him. If the player tries to run away through the spawn they will try to shove him back towards their boss.
If another player gets close enough to start attacking the mob that player will grab aggro. From just the encircled mob, not the boss himself.
I’m thinking in like last man standing scenarios, this would be a fun element to the game.
In general it would be nice to see some variety in the tactics.