Back when I used to regularly hang with fiction editors, they talked about a curious phenomenon. They’d get a book proposal that was new and completely unique…and then get 2 or 3 more in the next few months. I think the specific example they used was a deep sea diver and explorer of ship wrecks time traveling to the time when the pirate ship she’s exploring was still afloat, and she falls in love with the pirate captain and saves him from the battle that killed him and sank his ship. Totally unique, and they immediately bought it. Then got a whole string of book proposals with the same basic plot, none of which they could buy.

The moral was, when you get an idea, write and submit it as fast as you can, because other writers are getting that same idea at the same time. And whoever gets the idea across the editor’s desk first gets the sale.