Perhaps the reincarnated character has to face a situation where they are not fully accepted as Goku by his friends and family. They then have to prove themselves not just in terms of strength but also in terms of character, and in the process, they form new and deeper relationships with the Z - fighters.
One interesting plot could be a character reincarnated as a powerful mage in the Warcraft universe and having to navigate the complex politics between different mage factions. They might have to use their modern - world knowledge to outwit their rivals.
One interesting plot could be the dragon's struggle to adapt to its new form. For example, it might have to learn how to fly and hunt in a completely different way than it was used to in its previous life. Another plot could involve the dragon's interaction with other mythical creatures. Maybe it forms an unlikely alliance with a unicorn or has a territorial dispute with a griffin.
One interesting plot could be the reincarnated character using Gohan's latent power in unique ways. For example, they might quickly master new forms like Super Saiyan 2 and use it to protect the Earth from threats that were previously not faced in the original story. Another plot could involve the character's modern - day knowledge affecting how Gohan interacts with other characters. They could introduce new training techniques or ways of thinking to the Z - fighters.
One interesting plot could be a character reincarnated as a Pikachu and having to navigate the world of Pokemon using only Pikachu's abilities like electric shocks and agility. Another might be reincarnated as a rare Pokemon and trying to avoid being captured by trainers. Maybe also being reincarnated as a Pokemon trainer's pet and experiencing the bond from the Pokemon's perspective.
One interesting plot could be the protagonist using their modern - day knowledge to change Sakura's fate in the Naruto world. For example, they might help her become a stronger ninja earlier by introducing new training methods.
Some fanfictions might have a plot where Goku and Naruto are in a training - like fight. They exchange techniques and learn from each other. For example, Goku could teach Naruto about ki control, and Naruto could show Goku how to use chakra in a more strategic way. Their fight could be less about winning and more about mutual growth and respect for each other's abilities.
In some 'oc son of goku fanfiction', a common plot is the OC's struggle with his identity. He is the son of Goku, a great hero, but he wants to make a name for himself. So he might embark on a quest to find a new form or a new fighting style that is all his own. Also, there could be a plot where he has to deal with the jealousy of other Saiyans who feel he gets special treatment due to his father.
One interesting plot could be Goku having to balance his training and his relationships with the various characters in his harem. For example, he might have to deal with the jealousy among the girls when he spends more time with one over the others.
One interesting plot could be Naruto having to adjust to his new slime form. For example, he might struggle with not having hands in the traditional sense at first but then discovers unique ways of using his slime body for combat or daily tasks. Another plot could be Naruto using his ninja knowledge in the slime world. He could teach the slimes some of his ninja techniques, like how to move stealthily or use basic chakra concepts in their own slime - based magic system.
One interesting plot could be the protagonist, reincarnated as Misty, using her knowledge from the previous life to train Pokémon in new and unique ways. For example, applying modern training techniques to Pokémon in the Pokémon world. Another plot might involve Misty making new friendships with other trainers or Pokémon that she didn't interact with much in the original series. And perhaps she could also discover some hidden secrets about certain Pokémon species that were unknown before.