I would like to propose a modification to Moodle that allows us to have some courses show up in several categories in the course listings. Each course would of course still belong to one specific category, but one could have "symbolic links" to the course from other categories, very similar to how one can have symbolic links to files in different folders.
Because this would only affect the course listings and no other Moodle functionality it wouldn't be a big change and wouldn't have much effect on efficiency. It just takes some thought about what is the best user interface for creating and managing these symbolic links. It would be some extension of cvs:/moodle/course/category.php.