I have seen some posts regarding Geology and Geography, I would love to see one or both of these added to Khan Academy. Given that at the collegiate level many programs are moving towards a combined "Geosciences" department, combining geology and geography may be the most appropriate way to teach the material. All of these fields have high job prospects, especially geographic information systems (GIS), and are generally left out of K-12 education. Therefore, many young students come to university never being exposed to these fields and the many jobs available in these areas.

Note - I would be happy to contribute to these topics, currently an Environmental Dynamics PhD student.