"What do I do next?"

An aspect of composition that I'm currently fascinated with is development. How do you turn a simple idea into an entire piece? Coming up with new ideas is relatively easy. But once you have an idea, what do you do with it?It sounds like a pretty basic question, and too elementary for a professional composer to be pondering. But in many ways "what do I do next?" summarizes the entire craft of composing music.I have read many books about musical form. They usually all start the same, discussing the idea that all compositions can be broken down into some variation of an AB or ABA form. Then they prove this point with examples from history, usually with an emphasis on Haydn, Mozart and Beethoven.The most common criticism of all these methods is that there are millions of compositions that don't fit their neat and tidy models. To this they usually say "oh well", or excuse those compositions as "unusual".In many ways I find these studies of form useful. And when you look at a Mozart minuet you can easily comprehend "Oh, that's how this was structured." But when you look at even a Mozart sonata form, it no longer adds up. Sure, we all know that the classical sonata form starts in the tonic key, modulates the the dominant, etc. etc. The theorists go into great lengths to say "see, look at bar 29, that's the dominant key. It fits the formula!"But what the theory books never seem to address is how the composer got from bar 1 to bar 29 in the first place. Why did Mozart need 29 bars to get to the dominant, and not 16, or 8, or even 1? What is missing from all of these theories is a theory of development. A theory that says "when you have an idea, here's how to consider where to take it next."I'm not pretending that I know any better; I haven't come up with that theory either. It's a very complex topic, because one of the quintessential aspects of music is that there are no "right answers" but an infinite variety of choices. But it's a topic I'm very interested in. Perhaps one day I'll be able to form my own comprehensive "theory of development".