Actually, I have a reasonbly good idea of how it happened. I'd set myself some pretty strict limitations already; the game would have randomly generated dungeons, the game would be set in a single "haunted" castle, and there would only be the hero and possibly one or two NPCs to rescue as the only living characters within the game. However, I needed some sort of backstory, so I found some suitable "haunted castle music" (some fanmade version of the music from Super Castevania 4) to listen to as I did some chores and daydreamed about story ideas. Before long I'd already spotted the obvious loopholes in my restrictions (hey, in a fantasy game set in a haunted castle nothing says an NPC has to be living, right? [wink]), and my mind had constructed about six hours worth of non-linear action/adventure plot, with at least five characters and five massive "boss" fights in their early stages of planning. The word "epic" once again seems to be creeping into my game design. So far, it looks like this game will take a year, probably much more, in order to complete.
However, this time round I find that I don't really care. Most people here in journal land seem to have an impossibly ambitious project they are working on, so why not me? I'm fairly confident that I have or can acquire the skills to complete a game of this scope, since it's not based on impressive technology achievements. It's just a matter of time, and if I treat this as a "hobby" project it doesn't really matter if it takes me five years to finish, especially if I have fun in construction. Plus all the skills I learn will be directly transferable to smaller, more sensible projects.
So I guess this means I will be chugging away on the design document for a bit longer than expected. I should probably set myself a deadline of no more than two weeks from now to get the first draft done, so that this whole thing doesn't drag on forever (unfortunately I really need the design document to be semi-complete before I can esitmate properly how long I think this will take me). At the moment the design is several pages of random paragraphs tacked in an appendix labels "Brainstorming", so some reordering is required too.