The Chosen Ones

I think you could try getting inspiration from 'Impossible Bosses'.

Havent played it in ages. This is their wiki and forum.
http://portal.impossiblebosses.com/Home.html
http://impossiblebosses.wikia.com/wiki/Impossible_Bosses_Wiki

Their bosses had a rage meter. So if you didn't cast spells that reduced their rage, it slowly built up till they casted an ult. Kind of like your bosses now. But they had interesting other aspects.

Fire boss
The whole map goes dark except for a little globe of fire. You need to follow it around and stay in the light, or else you lose hp. It moves around rather randomly.

Nature boss
You had to kill his summons or else they would continuously keep healing him.

Rock boss
You had to run into portals that would teleport you around the arena to escape him during his ult (He kind of charges at you at high speed)

Can't really remember the rest though. There's like 9 bosses.