Okay, I have a question about similar concepts. Good God, no, not another Harry Potter rip-off. Fairies. Two or so years ago I began submitting queries for a young adult novel I'd completed. It happened to involve sinister fairies, which as you are well aware, now plaster every back cover of every young adult urban fantasy known to man. (Well, toss up between sinister fairies and sexy vampires). I was told time and again by editors that my writing was spot on and my book was infinitely publishable . . . except for the small problem that they already had several books like that about to published. I rolled my eyes and moved on to writing another manuscript, which I've just started to push out into the slush piles of agents and editors, with good response so far. My question is - what of Sinister Fairy Novel? Is it unmarketable now? Did my timing damn its chances? Or is there a chance that time will heal all wounds and I'll one day be able to market it again? Thanks for your time!
The children's book market, like most things, is cyclical. So yes, I'd say "sinister fairies" has a chance of one day not being so overdone.
It might even be sometime soon--though in editors' terms, "soon" can be something like 5 years. God, how time flies when you spend your days running around like a headless chicken.
If I had to guess (and it would be a wild guess; the market's unpredictable), I would say perhaps two years for most submitters to get tired of hearing "no more sinister fairies" and give up, and maybe another year to let editors feel like it's been a while since they saw anything about the evil fay.