Because I'm a sucker for a nice-looking avatar, I have a few suggestions:

* Themed costumes without massive amounts of tokens required to get them, like a green top hat for St. Patrick's day, a white Phantom mask with a sombrero for El Dia de los Muertos, and other holiday-themed items from all over the world. Perhaps the items could be distributed as random drops during the given holiday, with the likelihood of enemies dropping them increasing the deeper you go, and/or available for a high price in the Supply Depot. With the right drops rates, the items could still be rare enough to encourage players feeling less than lucky to buy CE and turn a minor profit for OOO (which I've heard is something they're interested in), and be in relatively high demand when they're up for grabs in the auction house.

* This, whether as a costume or actual armor, is too awesome not to have: http://malakym.deviantart.com/art/Doodle-Silkwing-Silks-330298254

* And, to finish this up, a return of the Deep-Sea Cuirass: http://wiki.spiralknights.com/Deep_Sea_Cuirass
* ...and the Jaws of the Megalodon: http://wiki.spiralknights.com/Jaws_of_Megalodon

It's not a lot, but then I'm relatively satisfied with the game at the moment. Thoughts?