Ability to add TAGS to articles.
And then see all articles with that TAG.
It would open up a way of pulling similar content together - a bit more flexible and extendable than the categories system.
If you have a page per TAG, listing the tagged articles, that might be good for SEO also.
May even be worthwhile having it optional in the top nav - with a page listing all of the available tags.
Not to replace the categories but to supplement them.
You might end up with 8 categories and 50 (or 500) tags for example.