Most of us working with audio projects have lots and lots of leftovers. There are few I know of that will release all the material they have put together, of many reasons. For example it simply does not sound interesting, or is such a different style so it’s hard to release it for many reasons.
It’s usually easy to know when something is not working out; if you work on the track for a longer time period, days, weeks, and is still sounds dull or not interesting, that’s such a track.
One school of thought is to just delete the material and go on. I think it’s a little bit drastic, but the idea has some merit. Time is short, and if you can’t rescue such a track, the statistics is against you. There could be a small possibility that later the track would work out. But if you have a long list of unreleased material, psychologically it kind of wears you down. So put them aside, into a separate folder or something similar.
The other option I use all the time is to reuse any samples, loops, drum material, and so on from unreleased tracks. I just export them to my sound bank, and there has been many times such recycled material shows up in unexpected configurations.
Another option is to export selected stems as possible raw material for something else, let’s say just the drums and bass. You could always later reuse it in combination with other samples, or as a starting point for a remix.
Finally, if you still really like the material, or can’t publish it of some reason, use it as parts of a personalized mix in combination with outside material, or then as exclusive private releases with published tracks. I do think such cases will be more and more common in future.
I should maybe go next and show my project folder setup for others to get ideas how to organize unpublished or published material.