I know it's still in the test build and hasn't been implemented yet, but I'm going to say it since it's still in development. Try to make the collections exportable to something like .OSC files or something. This will make it nice so people can share their favorite song collections or make training lists for others to use. Just my two cents, but this can be denied since it's not implemented but should be taken in consideration. There could even be a folder within osu! called collections where the files are automatically saved so no buttons would really have to be implemented, except maybe a 'go to the folder' one.