I'm guessing you probably don't solicit plugin ideas, but I have a thought anyway, just in case.
I get dozens and dozens of spam users a day who use spam links as their user homepage URL. Considering that users on my forum typically don't have homepage urls anyway, it would be nice if there were a plugin that would simply remove the homepage field on the user registration form, and then automatically reject any submissions that try to include a homepage URL anyway. I imagine this might actually stop my spam users altogether.