What I am thinking is a mxiture of oAuth and a profile manager. What this is is a way to have one profile that you can use on all sites that support logins with this version of oAuth.
I believe this can be very good as a user can log in to the main site update their information and view what sites are using their data. They can even choose whether the site can access certain data and have specific data for that site so from their name to the profile picture it uses.
A site wanting to use it would have to sign up to get a key etc. Then when connecting have to select what sections of data that they need (Name, Address, Email, etc). When the user registers it will tell them that the site wants to use this data and they can select whether they can use it. Similar to Facebook.