I am SHOCKED, that NOBODY has thought of doing a Supernatural/Monster Musume Cross Over. But since I am a fan of both series, will try and do something about that even if I am not yet ready to make my one story (one shot or multi chaptered lol)
So heres some suggestions/Challenges.
1. Have Sam and Dean Work with the MON Squad. Dean has a thing for Smith. Sam...either Manako or Tionishia.
2. Castiel is labeled as an Liminal, and is sent to live with Kimihitio and company.
3. Charlie is chosen as a host for Female Monster girls. Could either be the ones from the series, or OCs.
4. Crowley, making deals with various characters from the Show. Maybe have a history with Doppel and/or Lala (I could see it happening)
5. The Girls fall in love with the Winchesters.
6. The boys are responsible for finding new species, and determine whether or not their human friendly (example: Vampires or Levithans)
7. Gabriel sends the boys into the world of Monster Musume.
Just some examples. :)