Well, he did call him "C'thulu Deathwing". I'm pretty sure he'll look pretty damn awesome. And the fight sounds just great, reminds me of a Final Fantasy fight, where you defeat gigantic bosses part by part until you finally get him down. Should be fantastic! looking forward to this very much!!

Aye, after all Metzen hinted at the appearance of another old god in Cata, the one that was the main one responsible for corrupting DW... and since there won't be many big content-patches anymore, I guess we will meet him soon (or the idea has been trashed).
So my guess is that he will kinda merge with said old god, or it will turn out that the old god has been a part of him for a while now, kinda merged into him at some point in the past... so kinda like Knob's idea. Will sure be interesting!

If by "hint" you mean "drop 10 ton anvils", then yeah. We were told outright at Blizzcon last year that it's N'Zoth signing Deathwing's paychecks. Several large anvils have been dropped in game about it too (in Vashj'ir and again in Deepholm, just off the top of my head).

Then again, with how they inexplicably abandoned Neptulon's storyline, who the hell knows what they'll do.

Or... while writing up Mists of Pandaria they noticed a plot part where Neptulon's story would be getting the spotlight he deserves !

I'm calling it: Ole Nepto is the end-boss of the next xpack. Or he's Azshara's bitch.