First, you need to understand the original Hamtaro characters well. Know their personalities, likes, and dislikes. For example, Hamtaro is always full of energy and loves sunflower seeds. When turning them into humans, keep these traits. Second, create an interesting plot. It could be about their new human lives, like going to school or starting a new job. And don't forget to add some conflict, like how they deal with not being able to communicate with other real animals as easily as before.
To write a great Hamtaro human fanfiction, start with a strong concept. For instance, you could base it on a 'what if' scenario such as 'What if Hamtaro and his friends were humans in a world where small animals are endangered?' This gives a lot of room for creativity. Then, focus on character development. Even though they are now humans, they should still have elements of their original Hamtaro selves. For example, Bijou might be a very graceful human girl but still be a bit shy like she was as a Hamtaro. Also, make the setting detailed. Describe the human world they are in, whether it's a big city or a small town.