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.
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.
A possible fanfiction could focus on a group of Saiyans who are trapped in a dimension where they can only watch the Goku vs Frieza fight on repeat. They start to notice small details in the fight that they hadn't before, like the exact moment Goku's power level spikes or how Frieza is trying to adapt his strategy. This newfound knowledge could then be used by them in some way when they eventually break free from that dimension.
Trunks would also be interested. He has his own experiences with fighting powerful foes and would be eager to see how Goku, a great Saiyan warrior, fares against Frieza. Also, the younger Saiyans like Goten might be interested. They've heard stories about Frieza and would be excited to see their father or other Saiyan heroes in action against such a notorious villain.
Well, 'Goten vs Frieza: The Final Confrontation' is a really interesting piece. It shows how Goten uses his unique skills and training to take on Frieza. And then there's 'A New Hope for Goten Against Frieza' where Goten has to overcome some internal doubts as well as Frieza's powerful attacks. You can usually find these by simply searching on the major fan fiction platforms.
One interesting scenario could be a battle in a neutral dimension where Gohan's Saiyan powers and Naruto's ninja techniques are put to the test. Gohan's super strength and ability to go Super Saiyan might be pitted against Naruto's shadow clone jutsu and Rasengan.