It could be another powerful villain from the Dragon Ball universe like Cell. Since Gohan and Frieza are starting to form an unlikely bond, Cell might see this as a threat to his own plans of domination. He could try to break them apart and defeat them both.
One possible plot could be that Gohan discovers an injured alien creature that he decides to keep as a pet. However, Frieza somehow gets wind of this unique pet and wants to take it for his own evil purposes, like using its powers for his galactic domination plans. Gohan then has to protect his new pet from Frieza.
Obviously, Frieza would be a main villain. He is always power - hungry and self - centered. If Gohan has a pet with some special powers, Frieza would want to take it for himself or destroy it out of jealousy.
The fanfic could also delve into the character development of both. Supreme Kai might help Gohan unlock new levels of his Saiyan abilities. Maybe it would show Gohan teaching Supreme Kai about the unique aspects of Earth's culture in return. This interaction could add depth to their relationship and the overall story.
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.
One idea could be that Frieza, after being resurrected yet again, finds himself in a situation where he has to team up with Gohan. During their time together, they start to understand each other's motives and past traumas. Gohan's kindness and sense of justice gradually influence Frieza in unexpected ways.
One of the main themes is the clash of generations. Goten represents the new generation of Saiyans while Frieza is from an older, more menacing era. This creates an interesting dynamic. Survival is also often a theme, as Goten has to fight to survive against Frieza's brutal attacks. Additionally, the theme of justice can be seen, where Goten is fighting for what he believes is right against Frieza's tyranny.
One common theme is mentorship. Supreme Kai often acts as a mentor to Gohan, guiding him in his power development and understanding of the universe. For example, in many fanfictions, Supreme Kai teaches Gohan new techniques.
One common theme is the exploration of their different power levels. Since Gohan has Saiyan powers and Supreme Kai has his own unique abilities, fanfictions often play with how these powers interact. Another theme is friendship. They come from different backgrounds and yet find common ground, which is a great basis for friendship - centered stories. And sometimes, there's the theme of mentorship, with Supreme Kai guiding Gohan in some way.
Another theme could be power and responsibility. Gohan has great power as a Saiyan, and Supreme Kai has his own responsibilities. In the fanfic, they could grapple with how to use their powers and fulfill their duties in different situations.