Hmmm.. How about a KumoriCon Cosplay Calendar featuring images of Cosplayers from previous cons? It may be difficult to limit your choices to only the 12 months. You could always have a forum for voting on your favorite image and have people upload the images to vote on. You also have to get the permission of the cosplayers in the photos to use them for a calendar you are selling, but I think it would be one hot item if you had it.
Not sure it fits with a KCon booth, but tabi socks. I really want to see SOMEONE selling tabi. I know a lot of cosplayers that would just love to be able to buy some.
A KumoriCon Kimono or Yukata - Hard to say, would be expensive to manufacture, and difficult to design, but I havn't heard of any other con that had a kimono or yukata instead of a T-shirt or jacket.
Hmm.. May also consider a bathrobe for those who attend the "bedtime stories" panel.
A cloth book/manga bag might not be a bad thing either. It might make it more convenient to carry around your purchases, so people might buy it just for the convenience of having it.
The umbrella may be a good idea for the same reason. It does rain a lot in Oregon, and during the con is no exception. If you want a large umbrella... I know that a lot of cosplayers would want to take pictures holding it, and the implications of (gulp) sharing it.
Pantsu? I don't know. Wouldn't that start a rash of ecchi and hentai people going around trying to take pictures of KumoriCon underwear on all the schoolgirls and lolitas? Sounds like you are asking for trouble. Besides, I think ichigo, kuma, neko, and usagi are more popular and cute than our mascot. Of course you can sell them if you want to.