One idea could be a time - travel twist. Maybe Bulma discovers a way to travel to a different timeline where Trunks is older and there's a complex relationship that develops between them, full of emotional turmoil and discovery. It could explore themes of love across different versions of themselves.
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.
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.
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.
First, study the source material well. Know their personalities, their goals, and their relationships in the original story. For example, Bulma is intelligent and resourceful, and Trunks is strong - willed and determined. Incorporate these traits into the fanfic. Don't make them act out of character just for the sake of the relationship.
The main characters are Future Trunks and Bulma. Future Trunks is a Saiyan - half human, half - Saiyan - who is known for his strength and determination. He has traveled through time to try and change the future for the better.
A great plot twist could be that Baby Trunks is actually from an alternate future where things are very different. This would change the whole dynamic of how Bulma relates to him. Maybe in this future, Bulma didn't survive, and now he has come back to change the past.
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.
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.
One idea could be that Trunks discovers a new power while on a mission, and Bulma helps him control it in a unique way. Maybe she creates a device that can enhance his power but also has some unforeseen side effects that lead to some comical and heartwarming moments between them.