One possible fanfiction could be about Negasonic Teenage Warhead and Yukio going on a secret mission together. They might face some challenges that test their friendship and powers. For example, they could be trapped in an abandoned building filled with strange mutants and have to use their combined abilities to escape.
A plotline could be that they discover a new mutant who has powers that are linked to both of theirs. They have to work together to help this mutant control their powers and also protect them from a group of anti - mutant extremists. This would force Negasonic and Yukio to communicate better and combine their unique skills.
Well, it's hard to name the 'best' as it's subjective. But some popular authors in the fanfiction community might be those who really capture the essence of both characters. For example, there could be an author who's great at writing Deadpool's zany dialogue and Negasonic's cool indifference at the same time. They might have a really good understanding of the X - Men universe too.
One interesting fanfiction could be where Deadpool tries to teach Negasonic Teenage Warhead his unique sense of humor. Maybe he sets up pranks around the X - Mansion and she reluctantly gets involved. It could be a really comical take on their relationship.
In the comics, Negasonic Teenage Warhead's sexual orientation isn't explicitly stated as gay. Her character is more focused on her powers and adventures.