Well, it could be about Gohan and Naruto teaming up to fight a common powerful enemy. Maybe an evil force from another dimension is threatening both their worlds, and they have to combine their unique powers and skills to save the day.
It could be a powerful demon that has the ability to absorb the powers of others. This demon has been observing both Gohan and Naruto from the shadows and wants to take their powers to become invincible.
In a crossover with the DC Universe, Darkseid would be a great antagonist. His immense power and his desire for universal domination would put him at odds with Ultimate Gohan who stands for justice and protecting the innocent.
Perhaps Ultimate Gohan could cross over into the world of Star Wars. He could use his powers to assist the Jedi in their fight against the Sith. His ki blasts could be like a new form of Force power in that universe.
There were a few novels that could be recommended, including Those Years Living in the Future World of Dragon Ball, If Dragon Ball, Latiz took Gohan away, and The Strongest Sun Gohan of Dragon Ball. These novels all used Gohan as the protagonist, telling his growth and adventures in the Dragon Ball world. These novels might satisfy your need for Gohan to be the protagonist.
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.
Ultimate Agent 3: Ultimate Return was an action-adventure movie with Van Diesel as the lead actor. The film told the story of the confrontation between Cage and the villain organization led by Donnie Yen, as well as the story of Cage working with a powerful team of agents. The film was full of tension and excitement, presenting a visual feast of the ultimate return. The film was released in 2017 and was 107 minutes long.