HP: 20 - 40 Billion in 25 heroic (Assuming we fight him next xpac and inflation continues to be hardcore like it has been)
Location: Flaming Citadel (Assuming they make another ulduar or ICC type instance for him)
Ability 1: Flame Wreath - Because I heard Sargeras likes to troll people.
Ability 2: Defile - Casts it right after Flame Wreath
Ability 3: Twilight Slicer - Cast during Flame Wreath and Defile, just to make sure you can't possibly kill him
Ability 4 (On the offchance you survive the first 3): Blazing Heat - Ticks for 50,000 damage per second, stacking infinitely. Stacks every 1 second, and moving lowers the stacks.
I could see it happening.