What should my book be about this NaNoWriMo?

Yes, I realize that it is only August 22th. Yes, I know that there is still like 70 days until November, and NaNo 2011. But I felt so unprepared last year. I may have finished despite having NO planning, or really even a solid idea at the beginning. Here's the thing, though. I know that I could do so much better if I actually put some thought into it this year. Perhaps do some story boards or character sketches or something. Before I can do any of that, though, I need to think of what exactly I want to write about. Ive been playing around with a sort of Victorian love story gone wrong, and while I like the idea, I'm just not sure if I can get 50,000 words out of it. And since your not allowed to start actual writing till November, I feel like the idea will slip away between now and then. So I need something more concrete, something I can start building upon now so writing in November comes easier. Thoughts?