As I mentioned before, I got the idea for this story from a small fragment of an old project. I started writing 23 years ago, and I think I have every story fragment, finished and unfinished novel, and half-baked idea I ever came up with. Some of those ideas have been around since my very first DOS computer, which my dad gave me as a present for my Master's Degree graduation/first Mother's Day in 1989. They've migrated over six (or maybe more) computers, from DOS to Windows ME (ugh), XP, and 7, through a succession of hard drives, and from 5" floppies to 3" floppies, Zip disks, and now to assorted external drives and cloud backup. There's even a handwritten novel chunk, which I'm working on typing in. So I've saved all these novels and half-novels and idea fragments all this time, thinking maybe I'd get back to them someday. As I look back on them, there's some pretty good ideas in there. The writing makes my eyes bleed - I think I've improved in the last 23 years - but the ideas are good. A lot of those abandoned ideas aren't enough for a full-size novel, but the rise of independent e-book publishing has also seen a return in popluarity of the novella and novelette. No more discarding ideas because they don't fit into the arbitrary word counts required by conventional publishing.
I also still have the first novel I ever wrote, and a completed sequel which I had totally forgotten I had finished. I remembered the idea, but not that I had actually written it all the way to the end. These are on my list to evaluate for possible rewriting, revision and eventual release.
The point of this rambling, I guess, is to say, keep all your old ideas. Even if they don't seem like they're going to go anywhere, you never know what you might do with them in the future.
(Though I'll admit I threw away the original handwritten version of The Lost Book of Anggird once I completed the first draft of the new version because it was so bad.)