So, when you offer to "share" your idea with an author so that the author can do the work of writing and then share the profits from you, most authors will laugh in your face. (The others are too polite to do it, but trust me, they are tempted sorely.) Ideas may seem like the most important piece of any successful creative project, and while I know that a lot of authors say that ideas are cheap and that it's the execution that matters, I have seen a few projects where the idea alone seemed to hold sway and that the execution only had to be moderately good to draw an audience. That said, I don't think that it's the idea alone that makes it great. I think it's the combination of the idea and the person who writes it. It has to be the right idea for you, the author, in order for it to work. I think that a brilliant idea for another author will rarely be the one that makes me wish I'd thought of it first. It happens, but not often.
So, if you have a list of ideas for books that you want to work on, how do you choose the one that will give you the most bang for your buck? If you're early in your career, maybe you don't have a lot of ideas and you choose the one that is the most likely to match your skill level. This isn't a bad way of choosing ideas. Despite what people sometimes say about writing a book to stretch your skills, I think you can only stretch so much at a time. If you go out tomorrow and you really want to run a 6:00 mile flat and you've never run faster than 10:00 per mile, I just don't think wanting it is going to work. You're going to spend a lot of time being frustrated, and you may be better off setting a more reasonable goal to start with.
So, you can put the more complex idea onto your list of books that you will get to. One of my writing friends once set a goal to write a young adult book with 10 different narrative points of view. She spent five years working on the book, and it is still stretching her mind. But she also worked on several other books that didn't require quite as much writing technique and were more about story. I'm not trying to suggest that a novel that is more narratively difficult to write is actually a better novel, either. Sometimes novels told in the simplest way are wonderful. But a complex story idea may draw you to it because you are perverse and you like a challenge. Nothing wrong with that. You can start it, but if you set it aside to work on later, you are not giving up.
A second way to choose the next story idea is to look at the market at the time. Now, this has both advantages and disadvantages. One of the problems of writing to the market is that the market is constantly changing, and you are behind the curve. By the time you notice a trend, it will take you a couple of years, even if you have already finished a manuscript, to get it published and out to print. By then, the trend may be over. For instance, I love The Hunger Games, and wrote my own dystopian novel right after the first book came out. I am still working on revisions, and at this point, I don't know if the book will ever be published because editors, agents, and readers (including myself) are tired of dystopians and will reject even a good one just because it feels too much the same.
The advantage to writing to the market is that you may end up with more readers than if you write something that is closer to your heart. If you can meld what the market seems to want with what you want to write, then you are in a sweet spot. When I wrote Mira, Mirror in 2004, I knew that there was a retelling of fairy tales trend. I hoped that I could get on board that train, but my retelling was really different than other ones. It was told from the point of view of an inanimate object (the mirror in the Snow White fairy tale) and the plot took place almost entirely one hundred years after the evil queen dies and Snow White gets married. A few readers have complained that it isn't really a retelling, but a complete revision of the original, and that's fair enough. But I think a lot of readers enjoyed how different it was. The retelling aspect got them to pick it up and try it, and my good storytelling got them to keep reading.
A third way (and in my opinion the best) of choosing your next idea is to find what you are uniquely qualified to write about. I am always surprised at how few writers can actually tell this about their own work (and I don't except myself from this generalization). We don't see ourselves very clearly, and we often don't see what it is that makes us different and interesting. I spent years trying to write literary young adult fiction and eventually got good enough to publish The Monster In Me, my first novel in 2002. But I don't know if that novel displays some of my unique quirks in the way that other books of mine do. When I wrote The Princess and the Hound, for instance, I was able to combine my love of romance novels (and my expertise after having read thousands of them in my youth), my interest in fantasy, and my love of language that came from getting a PhD in literature from Princeton University.
Another writer I know is a gourmet chef, and I'm convinced that some of her best writing is about food, and that food needs to be a part of everything she writes, because she has a passion about that. Another friend is an expert in post-Roman Europe and is working on a fantasy that parallels the exact history of that time period. I love this book because I don't know anyone else who could write it. Most academics are too snotty to bother with fantasy and most people interested in fantasy don't know the details of this largely overlooked time period. Another friend has a handicapped child and she writes the most eloquent fantasy ever about people who have been magically handicapped.
What is on your list that only you can write? What is a combination of things that you are uniquely qualified to write about? I'm not against doing research, but a lot of writers already are experts in a certain area, and there is nothing wrong with using what you already know. You are also passionate about it, presumably, or you wouldn't have worked on it before you became a writer. Kathy Reichs, the author of the best-selling and long Temperance Brennan mystery series (on which the TV show Bones was loosely based) is a forensic anthropologist. She's an expert in her field, and I don't think anyone else can make pages of forensic anthropology as interesting as she does. She talks about bone fusion as people age, about diseases that change bones, about different populations with different bone shapes and sizes -- all with an expertise that I would spend forty years trying to catch up on. So I don't. I let her do it and enjoy her books. And I write my own mysteries about my own world: Mormonism.
I can't promise you that if you choose this way of sorting through your ideas for books, you will definitely make money. I can't promise you a lot of readers. It may be that your idea is too unique or that you'll die and your books may suddenly become best-sellers then, when people see how brilliant you are (don't we all hope for this?). But you will be writing books that you will be happy with, and I think that is more important than other considerations. Of course, I can't make decisions for other authors. Ultimately, your choice of the next book is entirely up to you. But choose wisely!