Okay so I was bored last night, thinking about how not even a drop of information has reached these forums for awhile.... Sorry almost went off on a rant there, but anyways I thought of the idea of transforming bosses, who have levels or layers of life. It's a bit difficult to explain so I'll make this little scenario:
You have a quest in which you must destroy a Triune Temple, after going down through the cellars of the temple you find it's "High Priest" in a set static area, he looks like the other Triune cultists, but is decked out with more flamboyant robes and stands much higher over the ranks. He's guarded by twenty or so cultists and ten Morlu (Assuming there in game). As you fight him and his minions he cast various buffs and various magic missile attacks (Not the actual magic missile). An X amount of Morlu come through two doors in the back of the chamber every X amount of time to support the Priest.
The Priest's life though is not much more than an average randomly spawned boss in the field, meaning that it would only be relatively difficult killing him if you can clear a path to him. As soon as his life bar is gone though it doesn't end!... Haha evil I know...
There is a short scripted event in which the doors that lead the Morlu into the chamber collapse and the chamber shakes, the priest drops to the ground and becomes momentarily invulnerable as he then rises into a form similar to that of an activated vessel, but larger, darker, and all around more aggressive. No more minions are in the way and its more similar to an actual super unique boss fight. This time he uses pure melee and will chase you about the chamber like Duriel after a lvl 9 sorceress.
A new full health bar is now on the screen, this time his hit points are monumentally increased, though his attack is relatively similar to a normal activated vessel or other strong melee monster, so still eventually, and probably without dieing you'll take his life bar back down to zero. Keep in mind you gain experience each time he is brought down to zero.
This is when another small scripted event occurs, the priest is engulsfed with fire, the chamber halfway collapses and then he emerges as a full fledged demon. (Note: The back Story does not need to be evident as this is a mere form my idea could be incorporated) Towering and dark its similar to... ...Lets say a Goatman mashed together with a... Crater Assault Trooper and Half the size of the siege breaker... (Don't mock me for my demon's hideousness!)
This time his health and damage is the classic through the roof style of Diablo 3 bosses (Well Sorta). He uses black novas and ranged attacks as well as strong knockback melee. After you kill him you get the loot and whatever quest may have been tied into him.
So roughly the idea is that a single boss can be difficult for every kind of character, different aspects changing and being difficult at different times. The concept is different than how the Thousand Pounder glows as he nears death for anyone who wants to bring that up... Secondly I think it adds more interaction with the bosses and increases the odds of memorable events. I don't know if the idea has been thought of before so I'm just throwing it out there. If you feel it adds any other pros or cons please post them up. :D
I just noticed how big of a monster this post is O.o....