For Yamcha, you can focus on his growth from being a bit of a comical character to a more serious and respected fighter in the context of his relationship with Frieza. Maybe he learns from Frieza's cold and calculating nature how to better strategize in battles. For Frieza, his development could be about him learning to care for someone other than himself. He could start to see Yamcha as an equal, which is a huge shift from his usual superiority complex.
One possible plot could be that Yamcha, known for his tenacity, somehow ends up in a situation where he has to team up with Frieza. Maybe they are both forced to face a common enemy that is even more powerful than Frieza at his peak. This could lead to an interesting dynamic as Yamcha tries to get Frieza to work as a team, while Frieza is his usual self - conniving and power - hungry. Their interactions would be full of tension and mistrust at first, but over time, they might develop an odd sort of respect for each other.
One idea could be that Frieza, in a strange turn of events, spares Yamcha during a battle and then gradually becomes intrigued by his tenacity. This leads to them forming an unlikely alliance against a common enemy. As they spend more time together, they start to understand each other's motives and personalities, and a unique bond forms between them.
In a fanfiction, their relationship could start as adversarial. Yamcha, being a good - natured hero type, would see Frieza as an evil force. Frieza, on the other hand, would view Yamcha as a puny being not worthy of his attention at first. But as the story progresses, it could turn into a relationship of begrudging respect. Yamcha might admire Frieza's power despite his evil ways, and Frieza could respect Yamcha's courage in the face of such a powerful foe.
One common theme could be the exploration of their complex relationship. Given their history as rivals in the original series, fanfics might focus on how they overcome that to develop a more intimate connection. Another theme could be the power dynamics between them, as Vegeta is often seen as a more powerful character compared to Yamcha.
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 the clash of good and evil. Trunks represents the good side, and Frieza is the epitome of evil in the Dragon Ball series. Another theme could be redemption. Some stories might explore if Frieza can be redeemed through his interactions with Trunks. Also, power struggles often come up, as both characters are extremely powerful in their own ways.
Yes, there are some. You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Trunks x Frieza' and you'll find a variety of stories with different takes on their relationship, whether it's a strange alliance, an unexpected friendship, or something more creative.