So Karvoth has a great idea: turn noncombat pets into combat pets (of a sort), and make them into a fun little minigame. Just like the Tonk wars (and doggone it, if anyone on Thunderhorn's Horde side has a Tonk left, come see my Shaman, because he has one and no one to fight with!), setting up a little mode for dueling minipets would create endless fun, and fill in all those hours of group waiting and guild setup pretty easily.
Of course, that's not the only game they could implement-- ever since the Darkmoon cards were implemented, I always thought it would be awesome to have an ingame card game. I probably doubled my time playing FFIX just because of Tetra Master, and putting a simple but addictive card game inside of WoW would give me a whole new reason to play even more.
Unfortunately, neither of these ideas are likely, because Blizzard has made it pretty clear with WotLK that WoW is some serious high-level business. But just like the new hairstyles, players will always jump at the chance to have more simple fun with all the time they spend in Azeroth.