If you want to hear the writing gods laugh...
If you want to hear the writing gods laugh, tell them your plans. Or better yet, tell your agent or editor your plans, in the form of the synopsis submitted with your proposal. Assuming that all goes well and you sell the thing, there with still be plenty of pages left to write. Pages in which your mind will run amok.
This happens to me frequently. I'll have a great idea all mapped out. Something simple and elegant that doesn't too badly strain my abilities. But after the sale, as I get to writing the novel, I start having better ideas. More convoluted ideas, for certain, which will be much more challenging to pull off but suddenly seem so much more interesting.
According to my editor (who is often surprised by my book's endings, which bear so little resemblance to the ones in the synopses), this is very common. So long as the marketing hook and premise remain unchanged and everything hangs together, no one in the publishing house gets upset about this. In point of fact, the ideas that crop up along the way are often the cleverest, most interesting portions of the book. Besides that, they serve to keep the author from getting bored with the "done deal," a book that has already been pre-imagined. They also serve to scare the complacency out of the writer and force her to seriously pay attention to what he or she is doing. Neither of which is a bad idea.
So as Joni (and her parable of the discovery of Lucy's elbow) recently reminded us, detours are opportunities to shake things up in surprising and exciting ways. So the next time you come to a detour in your writing, don't cling too tightly to the plan. Risk a little exploration and see where it takes you.
Have you ever struck pay dirt by exploring a wild hair in your own story? Or has the "wild hare," like Alice's, led you into trouble?