A really interesting plot could be that Broly trains Gohan in a parallel dimension where the rules of power are different. Gohan has to learn to adapt to this new environment while also dealing with Broly's tough training. This could lead to some really cool power - up moments for Gohan.
Well, the plot might involve Broly somehow getting stranded on Earth where Gohan lives. Gohan, being the kind - hearted Saiyan - human hybrid, decides to help Broly adapt. In the process, they train together. Gohan tries to teach Broly about control, while Broly shows Gohan new levels of power. Their friendship grows and they end up protecting Earth from other threats that come their way.
The fanfic might show that Broly, despite his usually aggressive nature, has a soft spot for Gohan's potential. He could see Gohan as a worthy successor to the Saiyan power. So, he trains Gohan with a kind of rough love. During the training, Gohan might struggle at first with Broly's intense regime. But as time passes, he starts to adapt and even surprises Broly with his growth. They could form an unlikely bond through this training process, which could be a central theme of the fanfic.
In a Broly x Gohan fanfiction, Gohan could develop a new understanding of power. Instead of just relying on his academic knowledge and some of his traditional training, he could learn from Broly that power can also be about pure instinct. Meanwhile, Broly could develop his mental faculties. Gohan might introduce Broly to things like meditation or mental focus exercises. As a result, Broly becomes less of a mindless brute when it comes to fighting and more of a thinking fighter, which would be a significant character development for him.
One reason could be the contrast between the characters. Broly is extremely powerful and somewhat of a wild card, while Gohan has his own unique power and personality. Their interactions in a fan - made story can be really interesting.
One possible plotline could be that Broly, in his rage, accidentally provokes Beerus. Broly, not knowing who Beerus is, attacks him. Beerus, initially amused, decides to teach Broly a lesson but soon realizes that Broly's power is growing at an alarming rate. This leads to an intense battle where Broly pushes Beerus to use his full power. Another plotline could be that Beerus hears about Broly's power and decides to test him. He travels to Broly's location and challenges him. Broly, seeing Beerus as a threat to his friends or his planet, fights back with all his might.
One possible plotline could be that Cell, seeking to prove his superiority, challenges Broly. Broly, in his usual berserk state, accepts without hesitation. The fight could start with Cell trying out his various attacks like the Kamehameha - like Solar Flare and his physical strikes. But Broly's power is just too much and he starts to overpower Cell. However, Cell then reveals a new, hidden ability that he has been developing in secret and the tables are turned for a while.
A plotline could involve Broly being sent to Kale's universe by some sort of interdimensional rift. Kale initially sees him as a threat and attacks. But as the fight progresses, they start to understand each other's power and motives. Or perhaps a fanfic could have a story where both Broly and Kale are training on the same planet. Their training methods clash, leading to a fight where they both learn from each other and grow stronger in the process.
A plotline could start with Frieza hearing about Broly's power and being jealous. He then devises a plan to ambush Broly. But Broly, being extremely perceptive in his own way, senses Frieza's approach and is ready for the fight. As the fight progresses, Frieza realizes that Broly is much stronger than he expected and has to use all his dirty tricks to try and win. Or perhaps the plotline could involve Broly being captured by Frieza's forces, and then Broly breaks free and goes on a rampage against Frieza and his minions.
There are so many fanfics out there, and to find good 'broly x gohan fanfic', you could also join some Dragon Ball - related forums. Fans often share and discuss their favorite fan - made stories there. Maybe someone has already found some great 'broly x gohan' ones and can point you in the right direction.