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.
In Gohan vs Naruto fanfics, a really cool plot is when their powers get combined somehow. For example, due to a magical or scientific accident, Gohan's ki and Naruto's chakra merge, creating a new and powerful force. Another interesting plot is when they have to fight in a world that has rules that are different from both of their own. This forces them to adapt in unique ways. Also, a plot where they are both de - powered and have to rely on their wits and basic fighting skills to compete can be very engaging.
In a Gohan vs Naruto fanfic battle, I think Gohan would win. His Saiyan heritage gives him a natural advantage in terms of raw power. He can fly and has energy blasts that could overpower Naruto. Naruto is strong, but his powers are more about ninjutsu and chakra control. Gohan's Super Saiyan forms would be too much for Naruto to handle.
Then, introduce some shared experiences. Maybe they get trapped in a strange dimension together. In this new and dangerous place, they have to rely on each other's strengths. Goku's physical power and Frieza's cunning could be the keys to their survival. As they go through these experiences, they start to see each other in a new light and their relationship begins to change.
Frieza's victims in the manga included many powerful fighters. For instance, he eliminated Dende. His actions were driven by his cruel and power-hungry nature.