You make an interesting point, actually. In TBC we had been looking at BT for a long time, sitting there in the east of Shadowmoon, right? And in WotLK, we were constantly told while questing in Icecrown that the Lich King was holding up inside ICC. Most of the quests there involved simply preparing the area for the Argent assault on the Citadel.

There's not really been a zone like that for Deathwing. Twilight Highlands does feel like the 'end-game' area, like Shadowmoon and Icecrown did, but Deathwing's presence is pretty minimal there - it's Cho'gal that pops up and threatens you and begs you to come and end his silly little mutated life.
Deathwing doesn't seem to have a 'home' or a base of operations or anything along those lines, because he's a badass and just likes to fly around blowing things up.

This makes me think one of two things - there will be a future raid just before we actually have to fight and finish Deathwing, that involves us fighting him to weaken him. At this point, he'll fly back to Deepholm where (unless i'm mistaken) he spent all that time resting after turning from Neltharion into Deathwing. The problem with this is that I'm not sure where in the zone he would go, and that Therazane and the dwarves wouldn't allow him back to rest so easily.

The other is that you progress through the xpac and find out about a new zone, and then head there to finish him off, maybe. Similar to the Sunwell in execution, perhaps.

Or maybe we all just hop on Alexstrasza and ride the dragon bus to wherever he's flying around torching things and stab him in the face. Conjecture, conjecture.