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.