One idea could be a crossover with the Naruto universe. Gohan as SSJ2 could end up in the Hidden Leaf Village and interact with the ninjas. Maybe he helps in a battle against a powerful enemy using his Saiyan powers.
Another possibility is a crossover with the Harry Potter world. Gohan in his SSJ2 form might land at Hogwarts. He could use his superhuman abilities to protect the school from dark wizards, and the students would be amazed by his power.
A crossover with the DC universe would be cool. Gohan SSJ2 could find himself in Metropolis and have an encounter with Superman. They could have a friendly sparring session to test each other's strength.
A really cool concept for an SSJ2 Gohan crossover fanfiction would be to put him in the world of Middle - earth from The Lord of the Rings. Gohan's strength and honor would make him a great ally to the Fellowship. He could use his super speed and strength to fight off orcs and other dark creatures. He might also have some interesting interactions with Gandalf, learning from the wizard's wisdom while also sharing his own knowledge of martial arts and ki manipulation.
A Mystic Gohan crossover fanfiction could involve the world of One Piece. Gohan could land on the Grand Line and join a pirate crew. He'd have to adapt to the strange new world full of devil fruits and sea kings. His powers would be a great asset, but he'd also have to learn about the different cultures and ways of the pirates. Or, he could cross over into the world of Harry Potter. Gohan at Hogwarts would be a unique concept. He could use his powers to protect the school from dark wizards while also learning about magic.
Another plot could involve Gohan being mistaken for a new metahuman in the DC universe. As he tries to find his way back home, he gets caught up in a conflict between the Justice League and a group of supervillains. His Saiyan powers could be the key to resolving the situation.
One idea could be Gohan getting transported to the Marvel universe and teaming up with the Avengers. He could use his Saiyan powers to help them fight against Thanos. Maybe his energy blasts would be a new weapon against the powerful foes in the Marvel world.
One idea could be Gohan from Dragon Ball crossing over into a world like Naruto. His harem could consist of female ninja who are intrigued by his Saiyan powers. For example, Sakura might be interested in his superhuman strength for medical research purposes, while Ino could be attracted to his calm and intelligent demeanor.
One idea could be a crossover with the Dragon Ball Super universe. Future Gohan could meet the present - day Goku and Vegeta. They could train together and learn new techniques from each other. Another plot could involve a crossover with the Naruto universe. Gohan might use his ki powers in the ninja world, and perhaps have to face off against powerful ninja like Naruto or Sasuke. He could also learn some ninja skills in return.
In 'Immortal Gohan Crossover Fanfiction', a great plot could involve Gohan crossing into the world of Final Fantasy. He could interact with the different races and use his powers to help in their battles against powerful summons. Another idea is that he crosses into the world of 'Avatar: The Last Airbender'. Gohan could learn bending techniques and share his knowledge of ki with Aang and his friends. He could also cross into the world of 'Lord of the Rings' and fight alongside the Fellowship against Sauron's forces.
A plot could involve Gohan being accidentally sent to a dimension where everything is made of lemon - scented substances. He has to find a way back home while dealing with the strange inhabitants of this world. He could learn some unique skills from them that are related to the lemon environment, like using lemon - essence to boost his ki attacks in a new way.
A crossover with the Naruto universe is also a possibility. Godlike Gohan could end up in the Hidden Leaf Village. His power would be a mystery to the ninja there. He could help Naruto and the others fight against a new threat, perhaps an ancient evil that was sealed away long ago but has now been released. His energy attacks would be different from the ninja jutsu, and he could teach the ninja some new ways of fighting.
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.