I'm not really into that kind of lemon - filled fanfiction. However, generally, such fanfictions might explore a more romantic or even spicy relationship between Trunks and Bulma. But it's important to note that a lot of this type of content might not be in line with the original spirit of the Dragon Ball series.
To make a Bulma and Trunks fanfic stand out, writers can focus on unique character perspectives. For example, showing Trunks' view of Bulma's scientific work from a child's perspective when he was young, and then as he grows older. Another way is to create original plotlines that haven't been overdone. Instead of the typical adventure story, they could write a story about a day in their lives where they have to deal with normal, everyday problems in a world full of super - powered beings. Also, using vivid and detailed descriptions of the Dragon Ball world, especially Bulma's lab and the technology in it, can make the fanfic more engaging.
One idea could be a story about Bulma's inventions and how Baby Trunks gets involved in her experiments. For example, he could accidentally activate a device that sends them on an adventure through time or to a different dimension.
One idea could be a story where Bulma invents a new device to help Baby Trunks train more effectively. For example, she creates a gravity simulator that allows him to train in conditions similar to different planets.
Some common themes could be family bonds. Since Bulma is Trunks' mother, stories might explore their relationship within the family dynamic. Another theme could be adventure. Given their association with the Dragon Ball universe which is full of action and adventure, fanfics might have them going on quests together. Also, character development is a likely theme, showing how Trunks grows up under Bulma's influence or how Bulma matures as a mother over time.
In some fanfictions, Vegeta is initially shocked. He might stand there in silence for a moment, his eyes wide. Then he could feel a strange mix of emotions, like pride and fear. He's proud to be having a child with Bulma but also fears for the safety of his family given his dangerous past and the many enemies he has.
One interesting Bulma Trunks fanfic could be the one where Trunks travels back in time and Bulma has to help him adjust to the past without giving away too much about the future. It's full of heartwarming mother - son moments.
I'm not sure about 'good' in the sense of lemon - themed ones as they often cross into inappropriate territory. But if you just mean fanfictions about Trunks and Bulma in general, there are many that focus on their relationship in a more positive and story - driven way, without the 'lemon' elements.
A fanfiction could focus on a day in the life of the family. Bulma could be working on some new invention in her lab, Vegeta is training hard outside, and Baby Trunks is getting into all sorts of cute mischief. Then, something unexpected like a small alien invasion occurs near their home, and they all have to work together to deal with it.
The story could explore how the news of Bulma's pregnancy affects their friends and family. Goku might be excited to have another Saiyan baby around, and Chi - Chi could offer Bulma some advice on pregnancy. Meanwhile, the Z - Fighters might be on high alert, fearing that the pregnancy could attract the attention of villains who would want to harm Bulma or the unborn child.