It was rumored that Illidan Stormrage, the neutral Legendary that never saw play, was being kicked out of Classic and being replaced. As it turns out, this is indeed correct!
Thanks to confirmation from Celestalon, we now know that Illidan will only be a hero and his neutral card is being renamed to Xavius. Here's what Wowpedia has to say about Xavius.
Lord Xavius was one of the Highborne's most powerful sorcerers and the high councilor to Queen Azshara during the time of the War of the Ancients. Since Azshara concealed her magical prowess, Xavius was considered the most powerful Highborne to whom the rest measured their skills. He was later transformed into the first satyr by Sargeras, before ultimately being transformed into the dreaded Nightmare Lord.
And finally, Celestalon's confirmation.
The neutral classic Illidan card got promoted from not-so-great Legendary minion to awesome baseline Hero… In his place, Xavius has taken over that design. (In other words, the old classic Illidan card got its name/art changed.)