Well, in 'gaster adopts a human fanfiction', it might start with the human character being in a world that is unfamiliar or dangerous. Gaster, who could be seen as a mysterious and powerful figure, notices the human. He might be intrigued by the human's nature or potential. So, he decides to adopt the human, which could lead to a series of events where the human has to adapt to Gaster's environment, learn about his ways, and perhaps they go on adventures together. It could also explore the relationship between the two, like how the human grows to trust Gaster and how Gaster comes to care for the human as a sort of parental or mentor - like figure.
One common theme is the idea of family. The dragon and the human child form a unique family unit, often defying the norms of both their species. Another theme is acceptance. The dragon has to accept the human child as part of its life, and the child has to accept living with a dragon, which is a very different kind of existence.
One interesting story could be about a powerful wizard who adopts a human child with special powers. The wizard teaches the child magic and they go on adventures together. Another might be a story where an alien adopts a human child and tries to understand human culture while raising the child in a very different environment.
The storyline could involve Rarity introducing the human child to the world of Equestria. She might teach the child about fashion and elegance, as that's her forte. The child could also learn valuable lessons from Rarity's friends, like loyalty from Rainbow Dash or kindness from Fluttershy.
Rarity would use her natural grace and poise to handle parenting. She'd start by dressing the child in the most stylish outfits, of course. And she'd likely be very strict about manners, as she values propriety.
One common theme could be the exploration of their relationship in the context of the outertale universe. It might involve how they interact with the unique settings and other characters in that AU.
There could be some. You might want to check out fanfiction websites like Archive of Our Own. However, the availability and quality can vary. Sometimes it takes some searching through different tags and filters to find the really good ones.
First, create interesting characters. The adopter should have unique traits and the child should be relatable. For example, if the adopter is a dragon, give it a distinct personality. Then, develop a strong plot. Maybe the child has a special mission and the adopter helps them. Also, show the growth of the relationship between the two.
I'm not sure if there is such a specific fanfiction. There are countless fanfictions out there, and while crossovers can be really creative, this one seems rather unusual. It would be quite interesting to see how the Naruto universe and the concept related to Gaster could be combined, but it might be a rare find.