Besides the fact that I'd be perfectly fine with that (humanity isn't very redeeming these days), I'd need to do something in WoW to mark the end. After all, it's been a constant companion for the last eight years. To mark it I'd do two things.
First, I'd plant myself at the hills overlooking Elwynn Forest and sit down. My first character would be my last -- my warrior. Even though he's no longer my main, I think it'd be fitting to go out with him. He was the character that got me hooked to the game and the people in it.
Second, I'd delete something. I'd pull out my Quel'Serrar, the original one from back in the day, and I'd delete it. Gone. Entirely. Kaput. It would be a dead polly. The only appropriate tribute to the end of WoW for me would be the deletion of the single most meaningful item I had back when things began.
What would you delete at the end? And don't say your character, because when the charred remains of the Earth are found by these guys, they might want to reconstruct WoW and see what all this fuss about CRZ and dailies was about.