Reminder that Illidan is blind, not deaf!
Here's some abilities he had in Warcraft III. Mana Burn, which burns an enemy target's mana, removing it while also doing the same amount of damage to them. Since Smash don't really do mana (Hero notwithstanding) Mana Burn would have to work a tad differently. There's also Immolation, which would deal damage to nearby enemies for as long as it's in effect. That'd have some interesting gameplay potential actually. Evasion, which gives him passive, well, evasion chance against attacks, and wouldn't really work as a Smash special move because everyone can already dodge. His ultimate WC3 ability, Metamorphosis, transforms him into a demon with a ranged attack and extra hit points. If translated directly to Smash, I assume it'd change his neutral attacks and buff his weight, but could just as easily be a final smash instead.
I can't speak about any possible gameplay potential from him in World of Warcraft because I only reached level 20 or so before I decided that WoW didn't really hook me.