I feel that a forum that would be not only interesting but also helpful to d2jsp members would be one based around fashion. Given the nature of this website, a sort of fashion advise section may be even more beneficial.
Not only could thins spark good discussion on current trends, outfit critiques, and other topics, but it could also help to aid in d2jsp members becoming better dressers. Because of the typical age of d2jsp's user base, this could be a great help for beginners wanting to dress better.
This section could be filled with basic guides (how to make sure clothing fits properly, examples of wardrobe stables, color/skin tone matching, etc.) as well as WDYWT posts (what did you wear today) for critiques of user's specific outfits and other topics on new lines, styles, etc.
I know many users with no interest in fashion will not think this is a good idea, but I am sure that there is a decent portion of the user base who are into, or would like to be into, fashion.