A reasonable list of providers is available but adding Auth0 will open the possibilities up. Auth0 is not only excellent but acts as a proxy to a very large list of providers. Plus they are just down the road to you guys :)
Important to me is the ability to use a custom authentication UI and their database storage. Otherwise I have to roll my own. My users do not have any existing accounts (including email) and so need a simple username / password experience (as horrible this might be for most of us)