When you have an idea that you can't make work on at the moment, what do you do with it? You should ALWAYS keep it.
Every writer has ideas that they can't use in their current work, but they all keep them for future use. Not because they could make a novel or even a short story out of them, but they can be springboards for other ideas.
Can't use that scene from a possible crime novel? Can you use the character in another story? Can some of the phrasing be used elsewhere? Is there anything in it that can give you an idea for something else?
The other advantage of keeping old ideas is that they can help you finish off a current piece by providing the link that you need, in much the same way as reading a novel can help you with a problem you're struggling with.
I have a folder of ideas next to my desk and some old fragments stored away on a file on my PC. They will not be thrown away. One day they could provide the answer to a question.