Well, this fanfic could be quite interesting from a character - development perspective. If Cell kills Gohan, it could change the entire dynamic of the Dragon Ball world. For example, Vegeta might be forced to step up in a different way, or the Z - fighters might have to completely reevaluate their strategies. It might also show a different side of Cell, perhaps a more menacing and victorious side that we haven't fully seen in the original series.
One possible theme could be the fragility of life. Even a powerful fighter like Gohan can be killed, showing that no one is invincible. Another theme might be the concept of revenge. If Gohan is killed by Cell, other characters could seek revenge against Cell.
Trunks could also be a popular character in such fanfictions. He might be curious about Gohan's interactions with Cell and get involved in their affairs. His time - traveling knowledge could add an interesting twist to the story. Also, Vegeta could be included. His pride might not allow him to accept Gohan getting too close to Cell and he could try to put an end to their relationship or partnership in the fanfiction.
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.
One idea could be to focus on Gohan's internal struggle during the Cell Games. Maybe he's doubting his abilities even more than shown in the original, and it's through his friends' and family's words of encouragement that he finally finds his resolve.
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.
In fanfiction, Gohan could have a secret power that he discovers during the Cell Games. This power could be related to his Saiyan heritage in a new and unexpected way, like being able to tap into an ancient Saiyan ability that no one else knew about. It could completely change the course of the fight against Cell.