The relationship could develop out of a need for companionship. Gohan, having lost many of his friends and family in battles, is in need of someone who can understand his pain. 18, being an android with her own complex past, also longs for someone to be close to. They meet and initially just offer each other a listening ear. As they talk more, they find that they can comfort each other in ways no one else can. This emotional connection then grows into a romantic relationship, and finally, they marry as they realize they don't want to be without each other.
One possible plot could be that Gohan and 18 meet in a rather unexpected situation, like at a martial arts tournament. They start to have conversations and find out they have more in common than they thought. Gohan's kind and intelligent nature attracts 18, and her strong and independent personality intrigues Gohan. As they spend more time together, they gradually fall in love and decide to get married, facing some challenges from their different backgrounds along the way.
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 such a fanfiction, perhaps future Gohan could warn present - day Gohan about upcoming threats that he has faced in his own timeline. He might share his experiences and knowledge, which could make present Gohan a stronger fighter and a wiser person.
Piccolo might also be a savior. He has a soft spot for Gohan since they trained together. He could use his strategic mind and powerful attacks to outwit Raditz and rescue Gohan. Maybe he teams up with Goku for an even more epic rescue mission.
If Gohan was a girl, the dynamics within the Dragon Ball universe would shift. Her relationships with other characters might be altered, and her power development could take a unique path.
Perhaps Gohan's own overconfidence could lead to his death. He might underestimate an enemy's power or a dangerous situation and make a wrong move that results in his end. For example, he could rush into a fight without proper preparation thinking he can handle it easily.
A possible plot twist could be that future Gohan is actually an impostor sent by an enemy to manipulate the real Gohan. Another twist might be that when they meet, their powers combine in an unexpected way that causes some sort of time rift or a change in the timeline.
Naruto would probably be the most interested. He loves making new friends and Gohan's power would surely catch his attention. He'd be eager to learn about Gohan's world and his abilities.