One common theme is identity. The brother raised by dragons often struggles to figure out who he is - is he more dragon or more human? Another theme is acceptance, both self - acceptance and acceptance from the Viking community.
One great fanfiction in this genre might be 'The Dragon - Raised Brother'. It shows a really interesting take on how Hiccup's brother, who was raised by dragons, adapts to human society again. The story is full of adventure as he tries to find his place among the Vikings while still having a special bond with the dragons.
The dragons that raise the twins are also key characters. They could be of different species, each with their own unique traits. For example, a Night Fury - like dragon could be very intelligent and stealthy, teaching the twins how to hunt and move quietly. And a Gronckle - type dragon might be more about brute strength, showing the twins how to stand their ground in a fight.
Another plotline might involve the twins' struggle to fit into human society. After being raised by dragons, they would have different values and behaviors compared to normal humans. This could cause conflict when they try to interact with their own kind. They might face prejudice from those who don't understand their dragon - influenced ways, but also find a few who are intrigued and want to learn from them.
Well, it varies a lot. Some stories have Hiccup being rescued by a kind dragon when he's lost as a child and then raised as one of their own. Another common plot is that he's intentionally given to the dragons by his parents for protection and ends up growing up with them.
There might be a fanfiction where Hiccup is raised not just by Toothless but by a whole group of different dragons. Each dragon teaches him different skills. Some dragons could teach him about flying techniques that are different from what he already knows, while others might show him how to communicate in more complex ways with dragons.
The dragons themselves are also important characters. There could be Toothless, who is often Hiccup's closest dragon companion in the original stories. And there might be other dragons like Stormfly, Hookfang, etc., each with their own personalities and roles in Hiccup's upbringing.
One plot idea could be that Hiccup, as a female, has unique ways of bonding with the dragons. For example, she might use her more empathetic nature to understand their needs better than in the original story. Another could be that she faces different challenges in the Viking society which is often male - dominated. Maybe she has to prove herself even more to be accepted as a dragon rider.
Her character development would likely be different because of her gender. She might face more discrimination from the Viking community at first. However, her time with the dragons could make her more resilient and determined to prove herself. For example, she may be more cautious in some ways as females in Viking society were often expected to be, but her dragon - raised side could make her bold when it matters.
You can start by looking on popular fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including 'How to Train Your Dragon' which 'hiccup raised by dragons' is related to. Another option is FanFiction.net. It has been around for a long time and has a significant number of stories in this genre.