As a writer of speculative fiction, the pressure of coming up with an original concept/ plot/ setting/ whatever constantly weighs on me, and in my experience, I've learned it just isn't possible. This is true of any genre, really. When working with new writers, I often hear the statement, "This is the most original idea I've ever come up with." And sadly, it's usually something that has been done before, not just once or twice, but so many times that you cannot even number them. All it requires is a little digging through the intrawebz to find your "most original" idea has already been used, sometimes decades before you, or perhaps your parents, were even born.
In science fiction, it is often a new technology or a scientific breakthrough, a concept that the author believes is so "out there" that it just might work, and there is no way in Hades that anyone else has ever thought of it.
Examples: Machines/ robots/ AI have gone astray from their intended purpose and plan to overtake humanity; through scientific research a miracle drug is created, allowing the human race to live forever in perfect health; an alien race wants to destroy us all… oh but wait, they're just misunderstood and actually need our help to save them from extinction… etc, etc, etc, the list is endless.
Try again. Please.
In fantasy, it is often a magical element, one that controls nature and/or physical objects, or something regarding telepathy. That is SO overused that I don't think I need any examples to prove the point. You've already thought of five or more by the end of this paragraph.
(I'm begging now) try again.
And when I say "try again," I don't mean, "try harder to come up with something original," I mean, "try a different approach because this current strategy of yours will get you nowhere fast." I need only mention the outrageous success of Stephenie Meyer's Twilight series to prove that an original idea is NOT what sells. Vampire romance? Been there, done that so many times that I just puked in my mouth a little. Yet the series is making millions.
So what's the catch? How can you create an engaging story out of a concept that's been done to death? To put it simply, it is not the idea that gets attention, it is how you present it.
In his latest book on writing technique, The Fire In Fiction, Donald Maass says it best: "What gives any novel the impact of the new is something that does not come from plot or milieu but from a perspective: yours."
Yes, that is YOU, the author. Many writers are afraid, although not consciously, to put their personal viewpoints into their stories through the eyes of their characters. They take well-intentioned advice that they should be writing with a specific audience in mind, and then they mistakenly write what they feel that audience wants to read. More often than not, readers can see right through this, and if that reader is a prospective agent or any other professional in the industry, you're in trouble.
The most important audience, in my opinion, is yourself. If you are not passionate about what happens in your story, how can you expect anyone else to be? So go ahead and write about the lowly wizard's apprentice that was prophesied to be the next great ruler of a kingdom currently oppressed by the queen of all that is evil, just make it your own.
How exactly do you go about that? That question has as many answers as there are writers. Everyone has different experiences, desires, lost opportunities, etc. Choose the elements of your own life that will enhance the story in a way that only YOU can create. Then watch your readers devour it because now, truly, they have never seen anything like it before.