You know how often wanna be writers use this line?
-- I have an idea, but I'm not sure if it's already been done.
-- I have an idea, but I'm not sure what will be selling when I finish it.
-- I have an idea that I want opinions about, but I don't want someone to steal it.
-- I have a great idea, but I need help with the plot.
This may sound insensitive, but if you're a writer, you write. Whether it's "been done" or whether the market shifts or whether you know the full plot when you start. If you want to write, then write it. If you're iffy about whether it's worth your time, it probably isn't.
No one is going to steal "your" idea. There isn't an idea out there that hasn't been done in some way already. It doesn't matter. What matters is how YOU do it. No one can tell your story like you can. So tell it. Or don't. But don't blame "but" and don't let it hold you back. If it holds you back, you may enjoy writing now and then but you're not "a writer" who will get anywhere.
Agents and publishers want books, not ideas. Readers want full stories. They want to read stories from authors who have been writing long enough and seriously enough to be good at their craft. They want emotion and depth and heart. If you have this and can put it into your stories and are willing to sit down for the hours after hours after hours writing and rewriting and rewriting more and cutting out your precious words that don't work well enough and filling in what hasn't been shown well enough and rewriting again and editing, editing, editing, and if you're willing to get critiques from those who won't pander to your ego ... and then go back and fix it ...
Then take your idea and CHARGE full steam ahead with it!
Someone recently asked me if the being unsure and rewriting ever stops. Being unsure never stops. Or it shouldn't. Rewriting eventually stops with each book after it's where it needs to be, but it starts fresh with each book that will need to be rewritten, also. They all will. The more you write, the less you'll need to rewrite because you learn more each time. A first draft, though, will always be only that.
I was noveling seriously for ten years before I put my first book out. That ten years included a novel writing class, studying the technique elsewhere, lots of reading (as I'd always done and if you don't, don't think you can write well), and TONS of rewriting. It's part of the job.
I have an idea ... actually I have lots of ideas, with seven novels in progress at the moment in different stages. Do I care what the market does or what anyone else is writing? Nope. I'm writing all of them because I need to write them. Hopefully, someone will want to bother reading them. Either way, they're going out there. No ifs, ands, or buts.