We'll need access to the addressbook and CardDAV is the protocol that is supported by the most providers (Or at least not the worst choice).

The first step will be to either figure out what implementation to use or write one. For the VCard implementation we should be able to use KContacts,
and at least a partial implementation of the DAV protocol should be available somewhere in kdepim-runtime, that we could either turn into a library or salvage otherwise.