I was torn between doing NaNoWriMo and Yuletide this year (practically speaking I could do one, but not the other), but I think I'm leaning towards doing NaNoWriMo.
Mostly because I feel like writing an original romance, and come November I'll probably have an idea that's fully 'cooked.' It's hard to describe why I want to write an original romance without sounding really arrogant: I think it will be a good use of me and my audience's time to write in that genre. Not because I think I'm god's gift to Romance (heavens no), but because I really like the genre, but I'm picky. There are a lot of common tropes that set my teeth on edge. And I am imagine they set other people's teeth on edge as well! So it seems reasonable enough to write a romance that I'd like, and hopefully some other people do to.
The Vampire/Undertaker idea I mentioned earlier could be fun, but I haven't come up with a plot yet.
I also have another idea: It's an arranged marriage plot, in a high fantasy setting. (Mostly because while I like arranged marriage stories, I like a very specific kind, where neither party is exactly thrilled by the prospect but they're going to make a good go of it, and then fall in love.) Probably M/F, but with both protagonists being bi, because it's my story and I can do what I want. And culture clashes! And being very noble and courtly! And animal companions! (So far the F!Protoganist has a lovely non-magical horse, and M!Protagonist is a sorceror-prince with a rat-king familiar-- who are maybe not a familiar befitting the prince, but are lovely themselves.)