I feel like the evolve concept would only work if they did it in low numbers, having 200 v behemoth sounds like a bit to much, and if there is no sense of reward that is balanced it could fall into the same hole as pvp.

I kinda feel like "evolve" concept could work if they did it in small number 4v, 8v, 16v, and 24v.

Though another balance would be how would enmity work. An idea that just came to mind is maybe the mob character could attack anyone but the dmg they do is based on the % of enmity.

So basically something like Tanks have 100% enmity so they take 100% dmg, but if the player mob attacked a character with 10% enmity they would only take 10% dmg which would encourage the boss player to focus on the player with most enmity however this enmity dmg reduction wouldn't influence skills used as the boss mob.

With the active abilities it could be like universal 15-30 second global cooldown with each ability having it's own extended cooldown to prevent spamming the same ability so maybe something like.

Skill A=2 min cooldown
Skill B=3 min cooldown
Skill C=2 min cooldown shares cooldown with D
Skill D=2 min coodlown shares cooldown with B
Skill E=1min cooldown
Skill F=2min cooldown
Skill G=5 min cooldown

So basically like some skills could share cooldowns to prevent them being used back to back within the universal cooldown or maybe even some abilities that open up the door to using other skills but the main idea behind this would be to balance out the boss mob without making them to OP against players because given the chance to be the mob players would aim for the healer first, and user their strongest ability everytime.