Export to XML, optionally with meta-data
The ability to export (and import) to an xml format would allow third party developers to be able to play nicely with the cards in this program. Cards could be exported to smartphone apps that understand the xml, or imported from that same app.
Additionally The cards have extra data, such as a category or tags and statistics. The ability to export this data in a granular fashion (selecting which data to export) would allow the deck organization and learning data to be transferred or synchronized with third party programs and apps.
Already implemented: see comments
A '*.card' file is essentially a zip file which another extension, which contains an xml file and the cards' media.
Granular export is also already implemented, since only active cards are exported.
Third part developers best work with the libmnemosyne python API, though, as generating the XML by hand is not recommended because of complexities involving the syncing algorithm.