1) Have to have the characters Yoruichi and Soi Fong
2) Has to take place before Urahara's exile
2) Have to be suitable for a T rating.
Other than that, anything goes. It can be romantic, naughty, one-sided love or mutual affection, humorous, angsty, slice-of-life, an UraYoruSoi triangle, what have you. Just send me a message (BebopSamurai) with a drabble and I'll get it copied on here; be sure to leave your alias or however you want to be credited on the drabb as well.
Oh, and you can contribute as many as you want-- just don't send in too many at once to avoid a build-up of the same author in a row.