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 common theme could be her early genius. Since Bulma is known for her intelligence, in baby Bulma fanfic, it might explore how her brilliant mind started developing even at a young age.
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.
I'm not sure about specific 'baby Bulma fanfic' stories offhand. But you can try searching on fanfiction websites like Archive of Our Own. They have a large collection of Dragon Ball - related fanfictions which might include ones about baby Bulma.
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.
As far as I know, no. The idea of 'baby bra' in relation to Vegeta and Bulma seems rather unusual. In the world of Vegeta and Bulma fanfiction, popular stories usually center around things like Vegeta's training, Bulma's inventions, and their family dynamic with Trunks. There's not much indication that a 'baby bra' fanfiction would be well - known.
One challenge could be protecting the baby from their enemies. Vegeta has made many foes over the years, and they might target the baby. Also, Vegeta's Saiyan nature might make him push the baby too hard in training, while Bulma wants a more relaxed approach. This difference in parenting styles could cause some friction between them.
Yamcha could potentially betray Bulma. Even though they have a history together, in a fanfic scenario, he could be jealous of Bulma's growing closeness with Vegeta or her scientific achievements. So, he might do something to undermine her, like spreading false rumors about her work.
Cell could also be an antagonist. He has an interest in powerful Saiyan genetics. The baby, being related to Goku, would be of great interest to him. He might try to absorb the baby to gain even more power, creating a big conflict for the heroes to deal with.
In such fanfics, Bulma might be portrayed as more adventurous in her love life. She could be shown as being open to new relationships outside of her usual circle, which might bring out different aspects of her personality, like her boldness or her ability to adapt to new situations.