A contact manager where you can:
-Edit contact information
-Link contacts
-Add extra detail not supported by the online services (eg. Phone#, Address, work, etc.) ** like seen in MSN Live Messenger
-Compare online profile with local profile for contacts

After some basic interface for that sort of thing is done, then maybe you could look into doing more advance things like:
-Link Addresses to Google maps or Live maps
-Link phone numbers to Skype
-Integrate "last seen"
-Birthday/Anniversary reminders