I like to say that ideas percolate. It sounds like something's bubbling. Sometimes they need to percolate for a long time. "Mull" is another good word. I'm mulling it over.
I've had an idea percolating about a short on Facebook for some time now. The thing is, I could never get the story right. I had the themes and the vehicle set, but I couldn't find the right plot. And then suddenly, last week, it just fell into my lap. Right from the front page of our campus newspaper. Now I'm really excited about the project. What few ideas I had before were forced and would never make a good short, for various reasons.
Sometimes writing is like finishing a puzzle. You've got one piece, and you've got to collect the others. So really, it's like a very complicated, time consuming, requiring of a scavenger hunt puzzle.
Check out the song "Whistle for the Choir" by the Fratellis. They're English. It's goodness.