One of my guild's officers has taken up the dubious practice of "farming for Wrath of the Lich King." He has epic mounts for all his 70s, all the recipes he could ever want, and enough spare cash to pay alt support for an army of gnomes, but he's convinced there'll be some big, 20K or so gold sink in the new expansion.
The gold sinks for WoW have always been epic mounts: the epic ground mount in vanilla WoW, and the epic flyer in BC. I'm not sure what it could be for WotLK. I can't see that many people would spend a ton of money on PvP structures, which leaves a couple of options. There could be extremely expensive forms of character customization, such as tattoos, wings, or other adornments. There could be guild housing (finally.) My guildmates suggested that it will be submarines shaped like "sharks with frickin' lazer beams" and underpants "with rockets on the front", further confirming that my guild's sense of humor is stuck in 1997.
What do you think the gold sink will be when WotLK finally emerges from its cocoon?