- Having a button called "Old", and then within that file structure having a folder for each course, with subfolders within that by topic. I'm guessing that this is what you currently have. This would work reasonably well if each course only had a few subfolders, but if there are a lot of subfolders in each then scrolling to subsequent courses would be awkward.
- You could add tags to the OPL taxonomy for your old courses. e.g. have a subject called "Old Problems", then a chapter with the name of each course, followed by a section for each topic within the course. This would involve tagging all of the problems, but could be automated by adding tags based on the names of the folders.
I think anything beyond that would involve upgrades to the Library Browser. I'd love to see the Library Browser improved so that it could handle a variety of different metadata, but that is a bigger project that no one has jumped on.