Best way to accept a fanfiction is to think of if as an alternate universe (which they usually are), or a different timeline.
I also have an idea for DD fanfic, but instead of doing it in Biscotti or Galette, it's in that world's equivalent of japan (Yukikaze and Brioche's homeland?).
The human OC would be a descendant of a "hero" that was once summoned into that country's war game and then sent back, bringing with him a method to return on his own.
But (many) years go by, and that person already died. His decendant finds in stored away in the attic/basement/whatever and accidentally activates it.
Ends up in Flonyard's "japan" (haven't thought up of a name yet) and meets a miko foxgirl
He becomes interested in that world...and you can guess the rest.