I saw some other post talking about Grom but it turned dumb. So this one is about something far more logical. Cenarius could come back because he is a demi-god. Dreadlord while not demi-gods can do the samething. Now Illidan became some sort of demon nightelf hybrid through consuming the skull of guldan and also possibly Tichondrius when he killed him. It is current belief the Tichondrius is actually dead, unlike Varimarthas and Mal'Ganis who could come back at anytime.
Now dreadlords lock away part of their souls in secret so if they die that can reform in a new body. With Illidan as paranoid as he was I get the feeling he would of done the samething. He had the Legion, Maiev, his brother and countless others after him. And with his move of saving some of the waters from the well of eternity I finding very reasonable to believe we will see Illidan again. But not as crazy maybe back to redeem himself.
Honestly sounds like a good plot device for the expansion after cata. Discuss