Let's just cut to the chase: we've had old gods, dragons, fallen semi-demonic elves, troll heroes, and soon the former prince of Lordaeron. Then comes the Cataclysm and we'll be looking to burn down the cthonian Deathwing himself, master of the deep places of the earth. A host of other luminaries will undoubtedly get in our way. Who would you like to see there? Who do you want to kill?
For myself, I'm looking forward to breaking off a piece of Ragnaros on the slopes of Mount Hyjal, but who I really want to kill more than just about anyone is Gul'dan. Yeah, I know he's dead: so what? Do a Caverns of Time: Sargeras' Tomb and show us that Gul'dan actually almost made it out of the tomb, bloodied but unbroken and with the Eye of Sargeras in his possession, and it's up to us future Horde or Alliance players to cut the guy down before he figures out how to use it to become godlike. Because as bad as the Lich King might be, can you imagine Gul'dan with godlike power? The guy stuck his apprentices' souls into jeweled truncheons and created the first DK's just as a means to an end! Gul'dan's conscience is so non existent it makes that whole "I pulled out my own heart so I would have no weaknesses" deal sound like a cringing emo move. Gul'dan never had a heart in the first place to pull out! This was a guy who deliberately sold his entire people into demonic bloodlust because it would make them less likely to ask him questions. So yeah, I want to kill Gul'dan. Make it happen, Chromie. Send me back. I have all sorts of pointy things I'd like to introduce him to.
How about you? Who do you want to kill?