First off, let me just say wow! I absolutely love your plug-in so far! You've done a pretty incredible job with it in a relatively short period of time.
I can think of just so many amazing uses for it outside of making a basic directory. For instance, we are a news site and I'm thinking of using it to create profiles for many of the companies and people that we write about in the news. A card on them could then be easily included using the appropriate shortcode at the bottom of the post.
That said, I originally stumbled upon your plugin when looking for a directory plugin. For instance, our goal is to create the most comprehensive list of poker rooms in Canada. In order to create a user-friendly system, it would be amazing if there was some AJAX interface that would enable the end user to browse categories, perform searches and generally just browse the whole directory based on various metrics - much in the way you can do manually in a post with the shortcode.
It would also be phenomenal to be able to expand the card into all the information available on a new dynamically generated page. If it was possible to tie in with the Google Maps API that would be even better.
There are a huge number of other awesome things that could be added to this plugin, but I'll have to restrain myself. I really do think that you'd easily be able to charge for a plugin that offered that level of features. I know that I'd certainly be interested!
Otherwise, I think that on the most basic level, throwing out a list of categories that users can click on in a static (non-AJAX PHP) page would be the most basic level that is needed to truly make a "directory". Hopefully we can get a feature like that soon!
Anyways, sorry for rambling, and really awesome work with this so far. I hope to see this plugin grow and evolve over time!