I was wondering if it would be hard to develop extensions that are category-specific. Like for example have a Music category where you could easily add a list of mp3's to a playlist when you post, which would then result in a little Flash mp3 player that accompanies your post.
Right now it seems to go against the grain of Vanilla, but I haven't really looked very hard at the code yet, so I might be wrong. :')
This should be do-able, but it might make life easier if your extension had some extra information in the category table.
I've been thinking that it might be useful to do the same thing with the role and category tables that I did with the user table - adding a "settings" field that stores an associative array of name/value pairs that extension authors can add to / remove from. Allowing you to customize those tables in any way you like without having to touch the database. I think I might put that in for the next rev.