We recently added some new functionality to the site. One of the things we added was a form, where you can request a new article.
So if you feel there is a need for documentation on a certain area, feel that there is something you don't really understand about Sitecore or if you just have a great idea for an article, fill in the form on the homepage.
The requests and our own ideas will be posted on a list here on the site, so it will be easier for article contributors to see, what articles are needed.