Metzen definitely confirmed Ner'zhul was dead.
Oh and my guess is that next xpac will bring us up to the space (we got ship, kind of stargate, magic its all possible there are dark portal that are open to other world/realm/planet) OR it will focus on heavy use of new time travel mechanics with player driven phasing (somethign in your bag to switch phase between dimension/time a la link to the past OR dark/light world from zelda TP). Now that they master phasing I am really sure they could do soemthign like this. They said cata would be amazing and so far almost everyone love the new 1-60 revamp, on beta people are saying godness about 80-85 zone, blizzard said next xpac will be on the same epicness scale.
Up to now the only thing blizzard hyped a lot was the pre cata event that failed but mistake happen uknow.
I just hope a time/dimension xpac or a space xpac Imagine goign on the white lady and fingding back the blue child and goin on it to discover what happened (why did the blue child dissapeared).