Gohan could also time - travel to a point in Saiyan history. He might witness the destruction of Planet Vegeta and try to save some of the Saiyans. This would completely change the course of the Saiyan race and his own story. He'd have to deal with the cultural differences and the power - hungry nature of some of the Saiyans he saves.
One idea could be to create a male OC who is a young warrior from a far - off land. He comes to Asgard seeking to prove himself and catches Thor's attention. They could go on a quest together to defeat a powerful dark magic that is threatening Asgard. The OC's unique fighting style, perhaps influenced by his own land's martial arts, could be a key factor in their victory.
One idea could be a crossover with Crash Bandicoot. Spyro could find himself in Crash's world, facing the same villains Crash does. They could team up to defeat Dr. Neo Cortex who has some evil plan involving both their worlds. This would bring a lot of action and humor as the two very different characters interact.
One idea could be a crossover with Ezio Auditore. They could team up to face a new, powerful Templar threat that spans across different time periods. Imagine the two assassins sharing their unique skills and knowledge.
In a fanfiction crossover, they could end up in each other's universes by accident. Ben finds himself in a world full of EVOs and has to rely on Rex to get back home. Meanwhile, Rex is intrigued by the Omnitrix and tries to figure out how it works. They face many challenges together, like dealing with Rex's old enemies who see Ben as a new target.
One idea could be that the Turtle Tots accidentally stumble upon a time - traveling device left behind by an enemy. They end up in the feudal Japan era. There, they meet young versions of their sensei and help him against some local villains. They learn about honor and the ways of the samurai, and when they finally return to their own time, they are more disciplined and better fighters.
Goku is often a common character. Since Gohan has a strong relationship with his father, in time - travel scenarios, they might interact a lot. For example, Gohan could go back in time to save Goku from an early death or to train with him. Vegeta is also a likely character. He might be skeptical of Gohan's time - traveling at first but then get involved in Gohan's plans. Another common character would be Piccolo. He has been Gohan's mentor, and in time - travel stories, he could offer his wisdom and help to Gohan again.
One idea could be Gohan traveling back to the Saiyan - Planet Vegeta era. He could try to prevent the destruction of the Saiyans by Frieza. Maybe he'd use his knowledge of future events and his advanced power to stand up to Frieza earlier. Another plot could be him going back to a time during the Cell Games. He could train his past self more effectively, leading to an even stronger version of himself to face Cell.
I really like 'The Time Traveling Lich'. In this fanfiction, the Lich uses time travel to try and rewrite history in his favor. It's full of action and has some really interesting twists on the characters' relationships. It shows how time travel could completely change the power dynamics in the Adventure Time universe.
One idea could be that Finn is kidnapped by an old enemy who wants to use him as a bargaining chip against Princess Bubblegum. Maybe they demand her to give up the Candy Kingdom in exchange for Finn's safe return.
Maybe Future Gohan travels back to a time before he was born. He could then try to influence his father, Goku, to be more prepared for the future threats. He might also give some advice to Piccolo on training young Gohan. This would create a butterfly effect, with many changes in the relationships and power levels of the characters.