One idea could be Goten traveling back to the Saiyan - Saiyan war era. He could end up changing some small events that have big impacts on the future, like uniting some Saiyan clans earlier. This would create a whole new timeline where relationships between characters are different.
Goten might time travel to a future where the Earth has been destroyed. He could then work with the remaining survivors to find a way to go back in time and prevent the catastrophe from happening. Along the way, he'd learn about the mistakes that led to the Earth's downfall and have to convince others to change their ways.
One interesting plot idea could be that Goten and Trunks travel back in time to the Saiyan - Vegeta and Goku's father Bardock's era. They could end up changing some events that lead to a different future for the Saiyans. Maybe they prevent Frieza from destroying Planet Vegeta in a more creative way than expected, like by using some future technology to make the Saiyans seem more powerful than they actually are at that time.
Another idea is Goten traveling to the time when Goku first arrived on Earth. He could interact with a younger Goku, Piccolo, and the others. He might accidentally change the way Goku grows up, for example, by introducing some new training methods he learned in his own time. This could lead to Goku becoming even stronger much earlier and changing the battles against the Red Ribbon Army and later villains.
Perhaps there is a situation where they have to team up to protect the Earth from a new threat. In the process, they discover feelings for each other that they hadn't expected. Their relationship then develops against the backdrop of this high - stakes adventure.
One idea could be that Goten and Trunks accidentally travel to an alternate dimension where their roles are reversed. Goten could be the more studious and reserved one while Trunks is the carefree fighter. They have to figure out how to get back while also dealing with the differences in this new world.
One idea could be a story where Chichi and Goten go on a training adventure together. They face various challenges and Goten learns important life lessons from his mother.
A Goten and Harry Potter crossover could also be cool. Goten could be sent to Hogwarts by some magical accident. He would be the odd one out, not understanding the wizarding world at first. But his Saiyan strength could come in handy during the Triwizard Tournament, for example, when facing the dangerous tasks. Maybe he'd have to use his ki blasts instead of magic spells to get through certain challenges.
One idea could be that Gohan is teaching a self - defense class and Goten pretends to be a beginner to surprise everyone. Another is that Gohan's class is a seminar on Saiyan history and Goten adds some new perspectives from his own experiences.
One idea could be Yoda traveling back in time to train Anakin Skywalker differently, thus preventing his fall to the dark side. Maybe he goes back to the very beginning of the Jedi Order to gain more knowledge about the Force and bring it back to his own time.
One idea could be Shinji traveling back in time to prevent the Second Impact. He might try to change the actions of his father that led to it. Another could be Asuka traveling to a future where the Evangelions have been completely forgotten and she has to figure out how to get back to her own time.
Another plot could be a character from the future coming back to the present in Shameless. They could be trying to stop a future disaster that is somehow linked to the Gallagher family. This would involve a lot of mystery and intrigue as they try to figure out what the future event is and how to prevent it while dealing with the chaos of the present - day Gallagher family.