Everyone wants to write the next big thing and if you do, then you need to write something that hasn't been written before. Now chances are that something similar has been written but you need to come up with something that sets itself apart from the rest of the books like it that have been written already.
When you read something amazing, you wish that it had been you who had written that book. You could write that book. Well, not the same one but you could also evoke that feeling in other writers but it's not easy.
You need an idea. And that idea has to have a unique spin on it. You need to be different. You need to stand out.
And that can be hard because you're worried you will fail. But that fear will stop you from writing that amazing book that you want to write.
So forget worrying about writing something different. Sometimes, different can be good.
No one has written about a group of lethal grandmas who roll like gangsters, with pistols sticking out of their heads. It may sound stupid, but people are going to remember it.
I do recommend working on an idea that works because I doubt you'll see a gangster grandma with a fake hip, limping around the 'hood with knitting needles hidden under her leather jacket.
When JK Rowling wrote Harry Potter, she wrote something that had been written many times before but she made Harry Potter different. She sent him to boarding school. She made the boarding school a castle. She made wands cool. She made glasses cool (I still hate wearing mine though).
When you sit down to write, make it your own and then one day, if someone else reads it, they'll wish that they wrote that book.