- There are extreme implications that the next expansion will feature the Burning Legion. Wrathion literally mentions it directly in 5.3. There have been hints since WotLK, and pretty heavy ones in Cataclysm.
- We're already exploring Illidan's story and showing that he wasn't as 'evil' as we thought.
- Blizzard has outright said that they want to bring Illidan back as a protagonist and 'redeem' him.
- Demon Hunter is the number one most desired class that hasn't already been implemented. (Just like Death Knights were, and then monks.)
Put together the pieces. It's not rocket science.