It could be that Gohan is kidnapped by a group who wants to study the Saiyan physiology. They might be scientists from a different planet or a secret organization on Earth. They take Gohan in order to run various tests on him, not realizing the danger they're in if Gohan manages to break free. And his friends and family then have to go on a mission to rescue him.
In this fanfiction, it could start with Gohan exploring an ancient Saiyan relic. Unbeknownst to him, this relic has the ability to bring back the spirit of Bardock. Once Bardock appears, he is initially confused about where he is and who Gohan is. Gohan, being the kind - hearted person he is, explains the situation. As time passes, Bardock starts to train Gohan, pushing him to new limits. Gohan also shares his knowledge of Earth's culture and values with Bardock, which makes Bardock see a different side of life compared to the brutal Saiyan culture he knew. This interaction could change both of their perspectives and lead to a new understanding between the past and present of the Saiyan race.
One possible plot could be that Naruto is sent to the Dragon Ball world through some sort of interdimensional rift. Since he has no family there, Gohan's family takes him in. Naruto, with his ninja skills and determination, could team up with Gohan to fight new threats in the Dragon Ball universe. He might also introduce some ninja - like strategies to Gohan and their friends, adding a new dimension to the battles. They could face off against villains who are interested in Naruto's unique powers from his own world, while also dealing with the regular Dragon Ball bad guys.
Well, the plot might involve Broly somehow getting stranded on Earth where Gohan lives. Gohan, being the kind - hearted Saiyan - human hybrid, decides to help Broly adapt. In the process, they train together. Gohan tries to teach Broly about control, while Broly shows Gohan new levels of power. Their friendship grows and they end up protecting Earth from other threats that come their way.
In the 'goku and gohan betrayed fanfiction', perhaps they are betrayed due to a misunderstanding. Say, Goku makes a deal with an unknown force to save the universe, but it's misconstrued as him allying with the enemy. Gohan, being associated with Goku, is also seen as a traitor. And this misunderstanding could be spread by a character with their own motives, like a power - hungry individual who wants to take over the Z - fighters' leadership.
One possibility could be that Gohan has been under a great deal of stress from his studies and superhero duties. Videl, perhaps not fully understanding the extent of his stress, keeps pushing him to spend more time with her or to be more involved in normal social activities. Gohan, at his wits' end, snaps at her, leading to a rift in their relationship. This could then explore how they both deal with the fallout, whether they try to repair the relationship or if it causes them to grow apart for a while before realizing their mistakes.
One possible plot could be Gohan going back in time to the Saiyan saga. He could use his knowledge of future events to prevent certain tragedies. For example, he might try to save the Saiyans from Frieza's wrath. He could also train with the younger versions of Goku and the other Z - fighters, sharing his advanced techniques and knowledge, which would significantly change the power dynamics in the battles against the Saiyans and later enemies.
Well, it could be that Gohan is being bullied at school and finally snaps. Maybe he's tired of the constant teasing about his intelligence or his differences. So he uses his hidden powers in a moment of anger and scares everyone.
Perhaps Gohan meets Zeno during a multiverse tournament. Gohan, being the smart and powerful warrior he is, could start a conversation with Zeno about the importance of protecting all universes, not just for the sake of power but for the life forms in them. Zeno might be intrigued and start to see the universes in a new light.