perhaps quick tags is a place where :
could be added for css nuts who like to markup their posts with custom css. an example being padding-left for an indent instead of using blockquote.
the ability in users or edit to transfer ownership of a post from one user to another. much the same function as categories.
wp-layout.css in the list of templates available to edit.
an non-table calendar that could be displayed inline and an available though perhaps commented out tag in the template for said calendar. a user could comment in our out the respective calendar without having to fiddle the template too much.
alternate styles for the actual wp interface, persons with reduced vision would tend to find the starkness of the design difficult to handle - i can whip up some css for this if it's considered interesting and or practicable.
wp is a desperately cool piece of work by the way.