I've heard, at several separate points in time, that Blizzard wants to/plans to bring back Illidan Stormrage. When and how obliviously have not been openly discussed, all I know is that Metzen original stated that the guys on the lore team love a good redemption story, and Illidan is about as good as it gets with that type of thing. More recently I remember hearing that Metzen and Crew have plans to bring Illidan back at some point.
I personally don't know how I feel about this, Illidan is my all time favorite Warcarft character, he's got a great back story and has had great character development, but I also hate it when Blizzard polls the "switcharoo" like this out of their asses (Kael'thas)(Bolvar/Lich King). How would you feel to have Stormrage back?