And as for the Thousand Pounder it is similar to my idea but a key point in my theory is that the entire AI of the Boss is changed, along with his attacks and model as he has "layers".
Your very right Boss fights should be epic. 8)
Sue me for making this comparison, but WoW does this all the time for bosses actually. It's still a great mechanic. If you watch WoW raids you will often hear people in vent say something about a phase 2 or phase 3. This is to note that the AI will change. Because of these changes the players need to know how to go about their play style weather they should focus on mana conservation or nuking the target (bringing it down as fast as possible) and so on. For example, a boss could be very resilient but weak on damage output in one phase, but then in the next phase he could possibly attack very hard but be more susceptible to damage. There are also other mechanics such as adds that would spawn in certain phases or specific abilities that the boss would get in a phase. It's not absurd at all to think Diablo will use this since WoW does. It's something that could work for any game.