I was thinking, following up this, we could have a "collectible card game" within WoW. Yes, I'm talking Yu-Gi-Oh style. As is, there are cards. Inscriptionists can make them. Mobs and bosses drop them randomly (the old decks). You could put decks together around stuff that already exists in teh game, so someone has a murloc themed deck and so on. They could tie cards in with the real CCG. It would be amazing, and if Battle Pets goes off well, that may be a next step.
And for those saying this is childish, I'm in my mid thirties, I played Pokemon back in teh day, and then later played it with my kids (both on GameBoy/DS, and the card game).