I reckon Deathwing'll be the final big uberwombleomgbossofdoooooooom.
It's probably gonna go;
- Kill Nefarion.
- "Look, I know he's burning the world, but I need you to go back in time again!"
- Old God, leading to Deathwing no longer being under his control,
- Deathwing himself.
If it goes that way, Blizz set themselves up for another crap ending like LK ("Father! Is it... over?" "No my son, these same 25 people will be back next wednesday")
I'd imagine Deathwing goes "Wait, I did WHAT?! I'm a bad bad person, I deserve to die... but the will to live is too strong, I must wipe you for a month in heroic mode before I finally kick the bucket!"
Failing that, we're potentially looking at a "My raid instance was merely a setback, I'll see you in the emerald dream next expansion!" from said old god.