I started Mulitboxing around the same time as the OP and I have been thinking the same thing. I have my main team in full t9 all pretty much around 5k GS and my Hunter and Mage alts are pretty close behind. I have over 30k gold in my bank and all the accounts and the shoulders and chest heirloom items plus a few weapons.
Now what I am doing is leveling a new team, the toons were sitting at early 60s for nearly a year. This team is now at 75 and I am looking forward to hitting heroics again at 80.
My last goal is to level a horde team from scratch to experience that content before Cataclysm.
All in all I have lots planned but I do get the feeling from time to time that I could easily just single box or even triple box and it is only when I am leveling that I really love the fact that I can walk into any area with my team and not have to worry about getting a bad pull and dying, instead a lot of times (Wintergard Keep is a prime example) I pull as many mobs as I can find just simply for the extra XP.