In Neil Gaiman's comic book series The Sandman, there is a library, presided over by Lucien the Librarian, which contains all the books ever dreamt of, including those that have never been written.
Most writers, I think, carry a dream library around with them, of books and stories that they will never get a chance to write, either because they won't have the time, or because the time to write those words has passed, or simply because it would not repay the effort, marketability being a factor in the decision about what to write next.
My own personal dream library is fairly large. I have no idea how it compares to my fellows.
I originally intended for Book of Shadows, my first novel, to be the first book in a trilogy, and, frankly, had I immediately set out to write the next two books, my career as a novelist would probably have been more lucrative. Fantasy trilogies sell better than photochemical smog science fantasy horror novels (i.e. my second novel SunSmoke). In BoS, the first of three magical devices (a magic sword) is destroyed, and it was my intention to take out one of the gizmos with each book. In the second book, whose title would have been Fire and Shadow, the ring would have been lost, when its possessor, well, basically ascends into the sky to become a star. In the third book, Shadows Merge, the protagonist, who by virtue (or vice) of heritage is completely immune to magic, chaperones the last device, a bracelet, to what amounts to a museum of lost artifacts, "never to be seen again," as the old Beyond the Fringe joke goes.
There are also two short stories that go into Fire and Shadow, one written, the other not. Will either ever see the light of day? Maybe. One's already written after all, and putting things up on the Web is cheap.
I also had a sequel in mind for SunSmoke: MoonMist, in which a magic spell descends upon Southern California, turning everyone into stereotypes. You may now make the usual joke: "Who would notice?" Well, among others, the protagonists of SunSmoke.
Paramount is never going to buy my Star Trek Novel, plotted as a lark one day in early 1980, and I've already mentioned what happened to my attempt to create a YA series for the Carmen Sandiego franchise. I'd be plenty happy to pitch my two Buffy the Vampire Slayer book ideas, but I think that pipeline is pretty well stuffed.
I'd have to seriously rewrite the first 20,000 words of Separation Techniques, as the terrorism and nuclear proliferation landscape has changed mightily since the mid-1990s, when I did the first part of it. At this distance, I suspect that I bogged down on it when it came time to kill one of the female characters, and I just didn't want to do it. Ben Sano tells me that I may be too kind to my characters, and he may be right. It's a little odd though; God doesn't seem to have any problem killing women in real life, so why am I bothered by doing it in fiction? Yet I am, and there it is.
I may someday get around to writing The Cuckoo, and/or The Zzyzyx, but until I do, they're also in The Taj Mahal Bookstore, which is what we members of the Albany State Science Fiction Fan Federation called the commercial version of Dream's Library, back in the early 1970s. In fact, I once write a story called, "The Taj Mahal Bookstore," which was not very good (another big reason why some stories never see the public eye), but did have one really good idea in it. The book that the protagonist finds is Huey, Dewey, and Louie's Junior Woodchuck Manual, from Uncle Scrooge comics. I later completely rewrote the idea, sans JWM, for a story called "Plot Device," that should appear sometime in the future, not sayin' where, I'm just sayin'.
Then there are the collaborations that did not pan out, "Lizard Run" and "Nightlife, With Gods," being the memorable ones. That's the biz, sweetheart. You'll read those sometime after the second issue of D'Arc Tangent goes on sale.
The stories I wrote when I was young are, almost without exception, godawful. I don't even remember the title of the one about the telepath sent to verify that the wolves on some planet are actually intelligent. Gah. I no longer have the original of the one that won the short story contest when I was 15, but even its later rewritten incarnations are slight to the point of blowing away in a stiff breeze. And I hope to god I never run across the novel I started in my teens, trying to emulate Harold Robbins. I honestly cannot begin to imagine what I was thinking.
There are a couple of stories I began in homage to Henry Kuttner and C. L. Moore, a Hogben story entitled "The Doosey," and a reworking of some of the ideas in "The Children's Hour" entitled "Shard of Darkness." I am the dark lord of retro SF.
Then, of course, we have the titles without much story to back them, my favorite being "Chimera Obscura." One is allowed to observe that I expend perhaps too much cleverness on titles.
Will I ever finish "Maxmillian on the Moon?" Who can say? It's about half done, but it's turned into a novella, and that's a bitch to sell. "Just Another Granny Death?" That may depend substantially on the fate of Dark Underbelly.
Like I say, I think a lot of writers carry around a Dream Library inside their heads. The universe of dreams seems larger than the one our senses clutch, but I can't say if this is good or bad.