When doing the Netherwing chain and the quests regarding the gronns in outland, I kinda suspected the deathwing expansion to be just around the corner. Having read War of the Ancients and multiple other warcraft novels, it's clear that Illidans story is connected to Deathwings story in some parts. Also, while questing in the burning crusade, I got many "hints" that deathwing actually destroyed Draenor. When I saw WotLK was the follow up to TBC, I said in my guild chat that Deathwing is next lots of times. When I was questing in Borean Tundra, I learned that Malygos was going down with the aid of Alecstraza, Kirin Tor (Rhonin) and krasus ofc. There was too many hints of the Dragonflights going after Deathwing in WotLK to believe even for a second that this expansion could be related to anything but Deathwing.
Also, the biggest hint in vanilla was killing his children imo. Why kill his children if Deathwing himself had no influence over them. The black dragonflight is not born evil as far as I know.
---------- Post added 2010-10-26 at 05:40 AM ----------
I believe you are correct in this assumption. The next expansion will definately focus around Azshara, there are too many hints of it already in the game to not assume that Azshara is the next big bad villain. I do think we will be visiting the eredars homeworld after that, with heavy focus on Draenei lore.