One idea could be that after the fusion, they have to deal with a new, even more powerful enemy that is threatening the entire universe. Their combined powers give them unique abilities that they have to learn to control quickly.
Vegeta would probably be both jealous and intrigued. Jealous because Goku has achieved a new level of power through this fusion, and intrigued because he wants to see how this new being fares against enemies. He might also be secretly glad that Frieza, his long - time enemy, is now in a form that he can tolerate (barely).
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.
Honestly, the best writers are the ones who can take the basic concepts of Goku and Fem Frieza and turn them into something completely different yet still in - character. Some writers do this by exploring different genres within the fanfiction. So instead of just an action - packed story, they might write a romantic comedy or a mystery involving Goku and Fem Frieza. These writers are often the ones who can surprise readers with their creativity and fresh perspectives.
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.
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.
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.
Power is a common theme. Since both Frieza and Cooler are powerful characters, many fan fictions explore how they gain more power, or how they compare their power levels. Another theme is family. Their relationship as brothers often plays a big part, like in stories about their rivalry or their attempts to cooperate. Redemption can also be a theme, especially for Cooler in some fictions where he tries to make up for his past actions.