There have been discussions about how to roughly categorize Moodle courses dating back to 2004 at least (https://moodle.org/mod/forum/discuss.php?d=16402) With the advent of learning analytics, it becomes more critical to have some way of categorizing courses so they can be grouped together for comparison purposes. Among the many taxonomies of subjects, this one seems relatively widely adopted, is intended for global use, and is specific to all types of education, including "initial education, regular education, second chance programmes, literacy programmes, adult education, continuing education, open and distance education, apprenticeships, technical or vocational education, training, or special needs education."
This taxonomy provides 25 fields of education organized in 9 broad groups (see section 286).
I think this taxonomy would be an excellent place to start to allow sites to specify the general nature of courses (level and subject). There could also be a site-specific "subject" identifier for local predictions (or this could be dealt with by allowing site-specific custom metadata at the course level), but it is critical to have a global taxonomy to be able to share predictions across multiple sites and countries, and for internationalization/localization purposes. This taxonomy could also be used to help identify potential competency schemae.
This taxonomy may apply to courses and students (and potentially teachers) as well as sites.
This classification should be optionally managed through presets during site installation.