One common theme could be adventure. They might go on adventures together and their relationship develops during those experiences.
One common theme is friendship. Since Pan and Trunks are from the same group of Saiyans, many fanfics explore how they support and rely on each other. For example, they might face a powerful enemy together and their friendship is what gets them through.
Growth and self - discovery are also prevalent themes. The characters of Pan and Trunks are still young and have a lot to learn about themselves and their powers. In many 'pan x trunks fanfic', they go through journeys that help them understand themselves better, and this is often intertwined with their relationship with each other.
A common theme is the adjustment to married life. They have to learn to live with each other's habits. Another theme is family. They might think about having children or dealing with their extended families.
One common theme is adventure. They might go on quests together and during that, their relationship blossoms. Another is the idea of Trunks helping Pan unlock her hidden powers, which leads to a closer bond.
Often, there's the theme of family influence. Given their family backgrounds in the Dragon Ball Z world, how their families' legacies affect their relationship is a big part of many fanfictions. For example, Trunks being Vegeta's son and Pan being Gohan's daughter, their families' reputations and powers play into the story. Also, the theme of friendship evolving into something more is quite common. They start as friends or comrades and then develop deeper feelings for each other as the story progresses.
One common theme is growth. They both grow as characters, whether it's in terms of their fighting abilities or their emotional maturity. For example, in many fanfictions, Trunks helps Pan to become a better fighter and Pan helps Trunks to open up more emotionally.
I'm not sure which specific 'trunks and pan lemon fanfic' is really good. You can try searching on some fanfic websites like FanFiction.net or Archive of Our Own, and look for ones with high ratings and positive reviews.
Rather than seeking out 'lemon' fanfics, you could try looking for general 'trunks x pan' fanfics on mainstream fanfic platforms. These might include stories that develop their relationship, perhaps in an adventure setting or during times of crisis in the Dragon Ball universe. This way, you can enjoy the relationship between the two characters without delving into inappropriate content.
A pan and trunks lemon fanfiction might focus on a time when they are both training together. As they train, they start to have a deeper connection. The story could explore how their shared experiences during training lead to a more intimate relationship, with their mutual respect and admiration turning into love.
A frequent theme is also friendship. Given their different backgrounds - Android 17 being an android and Trunks being a half - Saiyan - exploring how they become friends and trust each other can be really interesting. Some fanfics also touch on the theme of sacrifice, where one of them might be put in a situation where they have to sacrifice themselves for the other or for the greater good in the Dragon Ball universe.