I don't think it's just about how wacky and out-there you can get. The reason wacky and out-there stuff gets nominated is because some people have the reaction of "oh, fic for this would be so neat!" about just about everything, not just books and movies, but just random things you come across. I'm one of them.

I rarely end up writing stuff, but for example, I wrote Penny Arcade fic (another webcomic that's been on Yuletide for a couple years) several years before it showed up as a Yuletide fandom. Just because I had an idea and have enough friends who read Penny Arcade that I thought I would have an audience. But with really out-there stuff, I mostly think, eh, why bother? No audience.

Except Yuletide is your one time of year when you can see that someone else does actually want to read/write this source, too!