A lot of people try to figure out what makes a "good" book idea. How do you know if what you're thinking has the potential to be spun gold, or if you're looking at a truckload of future frustration? While this is a large and convoluted topic, here are a few basic points to consider:
Not every idea you have is going to be good.
And that's ok! In fact, it's an unavoidable truth. Some of your ideas will not be successful, and some will be downright ridiculous in hindsight. But just because you have a bad idea doesn't mean you'll never have a good one, or that you are a bad thinker or a bad writer. Creativity ebbs and flows, and so does the state of your ideas. Don't fall into the rut of "all or nothing" thinking; resist the self-talk that says if you're not drowning in advance money and publishing options with every pass of your hand over the keyboard, you're forever doomed to be a failure. Be patient and wait for the good ideas rather than barreling through with something iffy that has red flags. Find the courage to be honest with yourself about concepts that are sub-par, structurally unsound, or just not ripened.
What's an unripened idea? These are often identifiable if you're stuck somewhere in the middle: you feel strongly the idea has promise but you don't know what to do with it. Write these ideas down. Sit on it for a while, maybe even a few years. These flashes of brilliance have a tendency to reveal their true purpose over time, so it's ok to simply let something simmer in the back of your mind until it's good and ready to come out onto the page.
Are you being honest with yourself about why you like the idea?
We writers are overly emotional creatures at times, and we get attached to things. These things can be characters, scenes, objects, locations, concepts. If you love the project only because you are enamored with one specific element, your story will likely fall flat in time no matter how much you want it to work.
-Are you trying desperately to shoehorn in a superfluous character because you love the fact that he carries a bow with silver-tipped arrows? Be honest with yourself...you just love the idea of silver-tipped arrows (they do sound cool!).
-Are you writing a story solely because you love the time period in which the story is set? You're probably building a really cool world and doing tons of research, but are your characters doing anything original, or of real consequence?
-Did you start writing a character who has one particular element that you feel strongly about? Perhaps he has a disability that is close to your heart, or a characteristic that reminds you of someone specific in your life. In today's developmental editing world, I see a lot of this with political concepts and demographics. These are usually perfectly fine aspects to include, but they cannot define your character or the whole book entirely.
If you started writing the story because of a very particular element that you loved, you have some thinking to do. You'll have to find a way to detach from that obsession and continue to develop the book in all directions. You may very well be able to include that element or expand ideas around it, but it might not play as large of a role as you were hoping it would. Think hard about how much weight you're giving peripheral or decorative elements - they are rarely successful as the entire impetus for writing a book.
Don't write solely what you like to read; write what you're good at writing.
This is sometimes a strange concept for people. As an example, I frequently hear fantasy authors speak about the things they love about the genre, so why wouldn't they want to write it? "Because you're not very good at it," is sometimes the honest reply. This does not at all mean that person is a bad writer on the whole - but it does mean that their talents and particular voice are perhaps not suited for fantasy writing. I tried to write a fantasy book once. It was dreadful, but I convinced myself for a year that all it needed was editing. In truth, my storytelling style reaches its full potential in other genres, and I had to learn how to enjoy my own voice rather than try and make it something that it’s not.
I also hear many authors say they want to write like a particular author they enjoy. Straight imitation of someone else's style is rarely a good idea in writing. Unless you are a ghostwriter, your writing voice should be your own and not a diluted version of someone else's. For more information deciphering the difference between imitation and inspiration, click here.
None of this is to say that you will never be good at writing a style or genre that you love to read. The point here is to pay attention to your strengths and acknowledge your weaknesses in order to elevate your writing level. Can you practice and learn to write in new ways? Yes, absolutely. Will it sound natural and will the writing be good? That's up for debate and depends on you. If it doesn’t work then it doesn't work, no matter how much you might want it to.
Know the difference between a bad story and a first attempt.
One must also be careful not to trash a story too quickly. Just because your first draft is discombobulated and confusing doesn't mean the story idea is inherently bad. Sometimes the story is great, but the approach or execution is where the difficulty lies. Perhaps you need to tell the story from a different perspective? Perhaps a peripheral character is taking up too much page space and causing distraction? Perhaps the situations you put your characters in are too shallow or easy? Learning to allow your mind to be elastic and flexible in how it considers ideas is an invaluable skill. Put your idea on a mental Lazy Susan and see what it looks like from other angles, or even upside-down.
Some characters or elements could be in the wrong story entirely. Although they might be small, eliminating them can feel like pulling a rock out of your shoe. No one ever said you can’t recycle things, so pull problematic elements out of your work-in-progress and save them for another project. I always recommend keeping an "Ideas Journal" of sorts, where you can store all your favorite ideas in great detail and then pick them up again when you find the perfect home for them.
Are you writing something that's been done before, solely because you want to ride on the success of the concept?
This is a bit of a complex idea because market trends do matter. However, if you're simply trying to jump on a bandwagon, it's likely your story will not have many original and awesome things to propel it to success. Although formula or model writing is certainly a thing you can consider, such as in commercial fiction, generally you can't cobble together a bunch of popular ideas and strike oil. Your story still needs to have your own flavor to it, otherwise you're just flicking droplets of water into the ocean. How can you alter a well-worn trope? Can you combine multiple, unexpected ideas to create something new and fascinating?
Generally, authors should be more focused on what might break next in the market, not what's trending now. By the time you write the book, it will likely be long gone. It's true that everything in the world has already been done - but that doesn't mean we're excused of striving for creative and individual thought.