API to emulate Radio Access Cards using NFC. That way WP can be used as a universal access card. A lot of corporate users who have to carry their access card with them will appreciate it. It will eliminate the situation when a person forgets his access card and has to go to the security office to get a temporary one.
One possible application of such API is ability to configure the access card ID from Active Directory. That way IT administrators will be able to provision new access codes using familiar infrastructure.
It will also make a corporation more independent in choice of a hardware provider for their access cards. It will even allow seamless migration to new hardware. WP will be able to work with multiple access card formats simultaneously.