I was going to suggest a diary challenge! However my challenge was more along the lines of making the story be completely in the format of a diary entry. That could be a little restrictive though, so yours is probably better.

How about a hat challenge? Because there are quite a few hats in the show and I can't think of a reason not to.

I don't think a bondage challenge has been done yet and I can just imagine the porn potential.

Or if a bondage challenge is too specific, a kinky sex challenge would be quite fun. Because we could write about all sorts of obscure and/or bizzare sexual practices, as well as the obvious.

A flashback challenge could be interesting, with all or part of the story being told in the medium of a flashback. (Or flashbacks.)