A lot could be learned from World of Warcraft, they have quite good boss fights and they require tactics and cooperation from group. World of Warcraft uses boss phasing. Example of simple template:
Berserker King
Basic Abilities:
Cleave
Melee and others must care for their position, else they will get struck down by cleave.
Berserkering Shout
Increases damage dealt for himself and his allies by ??. Can take precaution by keeping him away from his allies.
Berserker Form
During Berserker Form he would change to ranged mode and reset threat. Meaning that you would have to stop dps for a moment to let the tank get aggro again. Resumes to normal form after 15 seconds.
Permanent Berserker Form
During Berserker Form he would change to ranged mode and reset threat. Meaning that you would have to stop dps for a moment to let the tank get aggro again. He would become more vulnerable to damage, but his adds would also become stronger.
Phase - 1
Until 80% Health then Phase - 2
Cleave
Berserker Shout
Phase - 2
Until 30% Health then Phase - 3
Cleave
Berserker Shout
Berserker Form
Phase - 3
Until Death
Permanent Berserker Form
Cleave
Berserker Shout
All phases
Allies would be coming to aid their master periodically that would require either kiting or off-tanking.
Everytime Berserker's ally dies, he gains Enrage that stacks infinitely, but expires after 6 seconds. Allies must be killed or they will wipe out your group. If too many Enrage stacks are placed at once, the tank will die to too much physical damage.
Hahah, I guess I have too much time because I'm writing these. But, couldn't this kind of simple stuff be alright? I mean this template has many things to be vary about.