This is a fantastic idea*: Take the Psynister's Ironman Challenge. The premise is to see how far you could go from a Level 1 character without the use of guilds, heirlooms, greens, steroids, plastic surgery, or a book deal: more Jake LaMotta than Lance Armstrong, if you will.
Think about it: you've leveled up almost every class. You've been through old content about a dozen times. You've nerd-raged over all the other idiots in the game because you know everything. Why not really challenge your skills? Do you have what it takes?
*And like most great ideas, I will have to back-burner it for awhile; however, it is intriguing. I'm still experiencing game roid-rage.