A decent shop hack. Or better yet, a decent cubecart integration hack they puts cubecart into your forums.
Cubecart is simple and clean with very few templates making it far easier to do this than with say x-cart or oscommerce. There are only about 3 or 4 template files. Essentially, get it to run the header and footer of the forums, tie the database of users together and bingo, full shop for vbulletin.
That, would be my number one request!
My Second would be a cool user profiles hack that is easy to install without needing loads of other mods installed etc etc