Users decide their role (within limits!)

We’re running a small multisite, where blogs are not as strongly connected to people as they are to ideas. So it is our view that when a user comes to read about an idea (on a blog) that they should be able to choose to ask to become a contributor or author and that this functionality should be integral to the site (e.g. via a wigit).
Thus users can become more heavily engaged with the ideas they like and just read about the others.

Any plugin available to do this, or any code in any current plugin which might provide a good basis?

Many Thanks,
Andrew.