Pumbz, you make some good points. And from the rumors I've heard posted on other forums, it sounds like there's an "out" for why players are already taking on Illidan; apparently he's actually killed by a fairly well known NPC, rather than the players themselves. That does make sense, because Illidan is pretty darn powerful, especially compared to the weakened lieutenants and old gods. Though, I guess you could argue that even Illidan is weakened right now, what with Arthas nearly killing him and all.

I hope they do put Deathwing in at some point, the character model is gonna look awesome. Also something to consider is like the Illidan fight it could be possible that you get npc help (in this case it would most likely be from the dragon flights) which from a lore perspective would be fitting. I also like the idea of the instance being in hyjal considering the instance entrance does look like onyxia's lair as some people have already stated