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.
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.
First, decide on the universe you want to cross Gohan with. Then, think about how Gohan would fit into that world. For example, if it's a modern - day Earth setting, consider how he would hide his powers or use them subtly. Next, come up with a basic plot like Gohan has to stop an evil force in this new world.
You could start by establishing the setting and the situation. Say you choose a crossover with the Avatar: The Last Airbender world. Begin with Gohan SSJ2 sensing an unusual energy in that world and deciding to investigate. This sets the stage for the story to unfold.
One common theme could be the struggle between Gohan's good nature and the newfound dark side. Maybe it involves his relationships being strained as his dark side emerges. For example, his relationship with his family and friends might change as he becomes more aggressive or unpredictable.