Good idea. I'm working at Nesna University college (HiNe) in Norway, an institution that educates a lot of teachers. From this autumn every professor and every student here will use Moodle, and my job is to teach them this. HiNe also has students in primary scools for several weeks, where they practise being a teacher. Moodle makes the contact between students and their practise school during the rest of the year easier, and even some primary schools want to start using Moodle.
This provides need for more than Admin, Teacher, Student and Guest. I'm thinking about something like Administrator, Teacher, Practice teacher, Student, Parent, Guest; which the administrator (and sometimes teachers) can choose which will be in use and in som level also to what they get access.
I'm not into any programming, but I'd really like to see this in a future version.