I'm thinking of writing fanfic for NaNoWriMo-- specifically the Tar Miriel/King's Man femslash I've had kicking around. I was originally planning to write something original, and I even have a couple of ideas, but they're not as solid as the Tar Miriel/King's Man fic.
It feels kinda like cheating to do a fanfic? But I know it's not-- and it's an investment in being more likely to complete NaNo. A fic where I have reasonable idea of what the plot will be, and which comes with pre-built ninjas* to throw at it if it gets stuck, is way more likely to get finished. And my original ideas aren't going to go up in a puff of smoke if I don't do NaNo with them! I can always write them later, assuming I'm not hit by a bus or something.
Plus, the planned fic will be a relatively high fraction original, considering it's a Character With No Direct Dialogue/An OC. So that'll at least be practice for original works.**
*Not literal ninjas. Literal Sauron though!
**She says, self justifying.