One possible plot could be that Goten and Future Trunks meet during a time - traveling accident. They initially have some misunderstandings due to their different upbringings and experiences. But as they start to train together, they learn from each other's fighting styles and develop a deep friendship. They then team up to face a new, powerful enemy that threatens both their timelines.
It could be a rogue Saiyan from an alternate timeline who has come to their universe to prove his supremacy. He is jealous of the power that Goten and Future Trunks possess and wants to take it for himself.
Well, there's a fanfic called 'Trunks' Time - Travel Tales' that's really good. It plays around with the concept of Trunks' time - traveling in a very creative way. Then there's 'Future Trunks' Love Story' which, as the name suggests, focuses on his romantic relationships. Also, 'The Legend of Future Trunks' is a must - read as it expands on his character and his place in the Dragon Ball universe.
The relationship could develop in a more emotional way. Maybe Trunks gets injured badly in a fight, and Vegeta realizes how much he cares for his son. He then becomes more protective and affectionate towards Trunks. On the other hand, Trunks could also start to understand his father better, seeing that Vegeta's tough love is actually a way of showing his love and concern. This new understanding could lead to a more harmonious and loving relationship between them.
One common theme is time-travel. Since Trunks is known for his time - traveling adventures, many fanfictions explore this aspect further, like dealing with the consequences of changing timelines.
She can be quite independent. In fanfic, female Future Trunks is usually shown as someone who can make her own decisions and follow through with them. She doesn't rely on others easily and is confident in her abilities. She might go on solo missions to save the day, which shows her self - reliance. Moreover, she may also possess a unique sense of style that sets her apart from other characters in the Dragon Ball universe, which adds to her charm in the fanfic world.
He becomes more resourceful. In his own time, he's used to a certain set of circumstances. But during time travel, he has to adapt quickly to new situations and find ways to use the resources available in the past. For example, he might have to figure out how to use different senzu beans or power - up techniques that are new to him.
You need to do a lot of research. Watch Dragon Ball Z to fully grasp Future Trunks' character traits and his world. And of course, be familiar with the MLP series. When writing, focus on the character development. Let the two different sets of characters grow and change as they interact with each other. Maybe Trunks becomes more in touch with his softer side through the influence of the ponies, and the ponies become more confident in facing danger because of Trunks.