[StarTouch]

I am 99.9999999% sure that Blizzard has stated in the past (if you ask me to dig up the source, I'm afraid I can't do that as it's lost in the god-knows-where regions of history) that World of Warcraft will continue where WC3 : TFT left off (storyline-wise). This is the reason I picked up WoW to begin with in the past. I wanted to know the aftermath of Arthas claiming the frozen throne and fusing with Ner'zhul.

My theorycraft on the matter (concerning the storyline) would be that if / when blizzard pulls out a Warcraft 4, they would introduce a whole new line-up of heroes / villains and MAYBE bring back a few "old" heroes.

Edit : While Illidan's Black Temple is indeed an impenetrable fortress as has been said, players actually enter it (lore-wise, through quests) as a distraction was provided by Xi'ri and the Naaru forces. This enabled Akama, Maiev and the players to happily skip into the Black Temple. By no means were the front doors busted wide open through an outright frontal attack.