If we decide we want to support diaspora* as a generic authentication provider for applications this could be a nice proof-of-concept and a little show-off project. However, given the complexity around that, I am not sure if we actually want that, and given the additional effort to implement an auth-with-diaspora based around dynamic client registration, I am also not sure if this is something applications would actually want. Even if we built something, I fear this will stay a very exotic project forever, and not gain much public traction, if any.
As for Discourse, the rate of GitHub signins here is very high, and that was somehow expected, as most people who participate in development related questions probably do have a GitHub account anyway. For others, signing in via eMail doesn’t seem to be too much of a hassle. I don’t see huge wins here.
That being said, if this is something you want to work on, I will certainly not block you, but there will be a lot of discussion involved.