There's nothing that says it has to be fanfic! It can be original! Well, you can't post in the collection at AO3 (or, well, they would rather you didn't, they can't actually police that all that well), but you can totally do it that way. There's not a lot of rules to a challenge like this.

And ten days of domestic fluff sounds lovely and it's quite likely I'll write another installment of my old!John and Rodney fic because this is just perfect for that sort of thing.