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.
One common theme is redemption. Frieza is often portrayed as having a chance at redemption through his interactions with Goku. For example, in some fanfics, Goku's unwavering belief in the good in people (or in Frieza's case, beings) starts to make Frieza question his own evil ways.
Another interesting one is 'Beyond Enemies'. This fanfic takes place after the events of the main series. Goku and Frieza are forced to interact in a new situation, like a cosmic tournament where they have to represent their respective races. It shows their growth and how they might see each other in a different light.
One idea could be that Frieza and Goku are forced to team up against a new, even more powerful enemy. They start off hating each other but gradually learn to respect each other's skills.
Well, friendship is also a theme. Despite their differences, they might form an unlikely friendship. In some fanfictions, they learn from each other. Goku might learn about strategy from Fem Frieza, while Fem Frieza could learn about determination and the will to protect from Goku. And of course, adventure is a big theme. They could go on quests together, exploring new planets and facing new challenges.
In some lemon fanfiction, their interaction can be filled with tension. Goku, being the good - natured Saiyan, may be initially wary of Frieza. But as the story progresses, they might find common ground. For example, they could start to respect each other's power.
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.