Interesting thread. I must say, my longeterm project, Non Serviam, is ALL completely off the top of my head and 'improvised', although I tend to get a lot of ideas when I'm drifting off to sleep, or just daydreaming. Funnily enough, I'm able to hold every last idea in my head, including the logistics and time-variant path-changing (i.e. switches, powerups allowing you to reach different screens, and what routes are available to the player at any stage in the game) to prevent sequence-breaking and the like. If something interests me, I have an almost photographic memory.
As such, apart from the major block I got about 2 or 1.5 years ago, when I simply couldn't bear to LOOK at NS in the editor, everything goes perfectly smoothly and just falls into place, and I always have an idea of what to create, or jobs to do, during my editing sessions.
My next project, which will be all content created by me (including music), will be a lot smaller, and more organized.