Every published author of my acquaintance has had this experience: Someone offers us, with great enthusiasm, their idea for a terrific book. (Invariably, they also offer to let us have half the profits from it, because that seems fair.) The idea may be from their life--of course, their particular life is unusually fascinating--or it may be a story concept they've been mullling for years but haven't gotten around to writing.
Most authors have developed their own way of responding to such generosity; mine is always, "Oh, gee, thanks, but I already have more ideas than I'll ever have time to write."
It's the truth. I have more ideas for possible books than I can ever get to. For me, this is the ideal situation. I have no anxiety whatever that I won't be able to keep creating stories. For some of my students, however, it's a recipe for brain freeze.
At a workshop I taught recently a very smart and motivated writer explained the dilemma. She had too many ideas, she said, and she couldn't focus on any one of them because all of them were demanding her attention. Another of my students sent me nine pages of the ideas doing battle with each other in his head; he was hoping I would be able to tell him which one to write. Some years ago I workshopped with a really, really good writer who found herself stymied by being able to think of a dozen ways in which a plot could go, and not being able to decide which way it should go. Her career floundered after one very good novel, and that is a loss to all of us.
The thing that the eager folks who want other people to write their ideas don't understand is that it's all about development. An idea is a great thing, particularly if it's a fresh, strong concept. Once the idea is born, however, the work has only just begun. It's like creating a garden. First, you choose the plot for your lettuce or flowers or orchard or whatever. Then you start to dig, and that's the real work.
Choosing which idea to use is a matter of prioritizing. There's no right answer. There may not even be a best answer, but there is an answer. No one can decide what that is for the individual writer. He or she has to choose the spot to start, and then--start. The screenwriter William Goldman quotes a theater director who said to a choreographer who was stuck, "Well, do something. Then we can change it."
The important thing is not to be overwhelmed by it all. We take small steps, every day. That's how a novel gets written. We inch forward, or leap forward, and sometimes we have to go back and take a different path, but it works out. One day we look back and think, "Wow! Look how far I've come."
If a plethora of possibilities really bothers a writer, she can make a list and keep it in a file. Lots of authors keep idea files for everything from great big novel ideas to scene ideas or even possible character names. Prioritize the list, and then start at the top. But do start!
And if you're one of those folks with a great book concept or a fascinating life, there's no need to pass it off to someone else. Write it down. You'll be amazed how good that feels.