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.
The fanfiction could be about Goku accidentally traveling back in time and meeting a young Grandpa Gohan. Goku would be shocked at first but then realize he has a chance to change some things for the better in Grandpa Gohan's life. However, he also has to be careful not to disrupt the timeline too much.
Perhaps in the fanfiction, Goku meets Grandpa Gohan in the afterlife. They could have a long conversation about how Goku has grown and how he's protecting the Earth. Grandpa Gohan might give Goku some new wisdom or advice on how to deal with upcoming threats, like a new powerful villain.
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.
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.
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.
In this fanfiction, Gohan might learn valuable life lessons from Grandpa Gohan. Maybe Grandpa Gohan would teach Gohan some old - fashioned martial arts techniques that have been forgotten over time.
In a fanfiction where Future Gohan meets Goku, it could be an emotional reunion. Gohan might be filled with a sense of relief and joy, seeing his father again. He could show Goku all the things he's been through in the future, and Goku would be both proud and sad about what his son has endured.
It could be a new villain created specifically for the fanfiction. Maybe a powerful sorcerer who wants to steal the Saiyan's power.
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.
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.
In the fanfiction, perhaps they team up to fight a common enemy. Goku's raw power combined with Gohan's strategic mind from the future battles could make them an unbeatable duo. They'd exchange blows and strategies, and their teamwork would be amazing to read about.
In this fanfiction, Bardock might be shocked to see how much Goku has grown and the family he has built. He could be proud of Goku for becoming such a strong and kind - hearted Saiyan. And when he meets Gohan, he might see the potential in him as well, maybe even offer some unique Saiyan training tips that are different from what Goku has taught Gohan.