It could also be that the writer has a different interpretation of the characters. Maybe they see some hidden tensions or potential for abuse in the original relationship that most people don't. But this is a very unorthodox view. In the mainstream view of Dragon Ball, Grandpa Gohan is a kind and loving figure to Goku. This type of fanfiction is likely to be very controversial and not well - received by the majority of the fandom.
I'm not aware of any such fanfiction. The relationship between Grandpa Gohan and Son Goku in the original story is one of love and mentorship, so this kind of dark and negative portrayal in fanfiction is not common that I know of.
One possible plot could be Goku reminiscing about his past with Grandpa Gohan. Maybe he visits the place where they used to train together. He could find an old relic or something that Grandpa Gohan left for him, which triggers a flood of memories and emotions. This would be a heartwarming story about their relationship.
Some people might create it just for shock value. They know that the original relationship between Goku and Chi - Chi is loving, so by creating something so opposite, they can get a reaction from readers.
Goku and Grandpa Gohan's interaction could be full of wisdom - sharing. Grandpa Gohan was a wise man, and he would pass on more of his knowledge to Goku. Goku, in turn, would tell Grandpa Gohan about all the new friends he has made and the different planets he has visited. They might also spar a bit, just like in the old days, with Grandpa Gohan giving Goku some pointers.
Perhaps in the fanfiction, Goku accidentally ends up in a different part of Otherworld where he stumbles upon Grandpa Gohan. Grandpa Gohan could then take Goku on a tour of Otherworld, showing him all the different areas and the souls there. They might encounter some old enemies or friends who have passed away. And along the way, Goku and Grandpa Gohan could strengthen their bond and Goku could grow as a character from the experience.
One possible interaction is that Bardock could spar with Goku and Gohan. He'd test their limits and see how much they've learned about fighting. Another could be Bardock sharing stories of Planet Vegeta with them, which would be really interesting for Goku and Gohan to hear.
The main characters are clearly Goku and Grandpa Gohan. Goku is the Saiyan hero who has had many adventures on Earth and beyond. Grandpa Gohan was the one who found and raised Goku when he was a baby. In this fanfiction, their meeting in Otherworld is the central focus.
It could also be a form of catharsis for those who have experienced abuse in real life, though this is a rather controversial way of dealing with it. By writing or reading about abuse in a fictional context, they may feel they are processing their own feelings, but it can also be triggering for others. Another reason might be that some writers simply want to create shock value to get more attention for their work.
The existence of such fanfiction might be due to the creativity of the fans. They want to experiment with different character arcs for Naruto. In the original series, Naruto is known for his non - violent approach and his goal to protect everyone. But in this type of fanfiction, by making him a killer, fans can explore the consequences of a character going against their core values. It can also be a reaction to the sometimes overly - idealized view of Naruto in the official story, allowing for a more gritty and real - world - like exploration of his character.
Maybe some people just have very unique and niche interests. They might find some strange fascination in the idea of Pokemon feet for reasons that are hard to understand for the general public.