How about, in a future patch of maybe Burning Legion expansion we fight...

Kael'thas Sunstrider... AGAIN.

And then Archmage Aethas Sunreaver, who's there to help us, reveals he's ALSO Kael'thas Sunstrider.

Basically Kael'thas mage magics were so strong, that when he cast Mirror Image, none of the mirror images knew who was the real Kael'thas and, unable to accept it was themselves, they all went their own way, mostly driven mad not knowing if they were real.

You killed one Kael'Thas in Tempest Keep

You killed a second Kael'Thas in Magisters Terrace,

Aethas is the third Kael'Thas

And now this guy is the fourth... and if he's the real one, Aethas will be no more once you kill him too.