Getting a nice web site/blog is not easy for beginners, even with cool predefined themes. That being said adding some quality pictures in pages can dramatically improve their appearance. But finding such quality images and using them without copyright infringement is not easy for novices. Therefore I would find very useful to offer a shared media library with pictures that every blogger could use on his website.
The global administrator could fill this library with pictures he would have chosen. For example, edublogs could offer a shared library with professional pictures related to education : http://www.istockphoto.com/file_search.php?action=file&text=education
Better : this pictures could be organized by categories. For example : School building / School administration / School activities / …
Another advantage of such a shared library could be to limit the number of files uploaded by users and thus reduce the hosting costs. Such a library could be called “Nameofblogplatform shared library” and the default WordPress library could be renamed “your files” (for example).
Would you be interested in developing such a plugin?