Pretty much every nanowrimo, I ask my girlfriend what kind of fanfiction she wishes she could find, and I write a novel sized fanfic for her. She mentioned recently that her favorite Jurassic World writer had deleted their blog (and with it, their stories), so that may be where we go this time. She's also been on a Once Upon a Time kick recently, so that is another possibility. I'd prefer dinos though.
I also have a folder of original ideas, but I'm not sure I have the time to flesh any of them out this fall while trying to plan surgery.