If you're in the mood for Supernatural gen, K Hanna Korossy on fanfiction.net. I don't think there's anything 20K, but there's a few 10K, and every one of the three hundred some-odd stories (bar a round robin here and a sequel to somebody else's fic there) is in the same universe, so really it's one biiiiiiiiiig fic. :-D

If you want sexy fic, sorry. I haven't got time to reread everything not mentally tagged 'blowjob' in order to find you-safe fic.