With everything that has been datamined so far, I have a theory as to what the Deathwing fight will consist of.
Phase 1
Simple tank and spank with a stacking debuff. This phase will require 2 tanks and sees players attacking the human form of Deathwing as is indicated on artwork released by Blizzard.
Phase 2
A semi enrage if you will, with Deathwing assuming his dragon form and overpowering the raid momentarily. We are saved by Alexstrasza and her red dragonflight. During this encounter, Deathwing raises his children Nefarion and Onyixa from the grave and they are tanked apart from each other so as not to receive the Buff. Each "add" is tanked away from the raid and the classic abilities Breath and Shadow Flame return. Once we have defeated his two children once again. We then aid Alexstrasza in his defeat.
Phase 3
The raid focusses on Deathwing with Alexstrasza still doing the majority of tanking, but requiring more intervention from the players in the killing of key mobs to weaken Deathwing. Adds will spawn from time to time and will have to be tanked and killed as soon as possible. With her help, we are finally able to overcome Deathwing. Loot dragon.
I'm not saying it's 100% going to be like that. Hell not even 1%, but in my opinion, the fight should be grand and I personally can't wait for it.