One common theme is the adjustment to married life. Given Goku's naivete in some aspects and Bulma's strong - willed nature, their learning to live together is often explored. Another theme could be how their relationship changes the dynamics with other characters in the Dragon Ball world.
I'm not sure if there is a specific 'Goku married Bulma' fanfic rated M. Fanfic content can vary greatly, and while there might be some creative and alternative storylines in the fanfic community, this particular combination might be quite different from the original story. It could exist, but it would be a deviation from the main Dragon Ball narrative.
A possible plotline could be that Goku and Bulma's marriage leads to a new era of peace in the Dragon Ball universe. With Goku's strength and Bulma's intelligence, they could start initiatives to better the lives of all the races in the universe. They might also have children with unique powers that are a combination of Saiyan and human genetics, and the story could follow their growth and development.
Well, it could lead to all sorts of interesting plot developments. Maybe they'd have super-powered kids or face challenges together as a married couple in the Dragon Ball universe.
Yes, there are quite a few. You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Goku and Bulma married' and you'll find a variety of stories with different plotlines and writing styles.
There could be new antagonists created specifically for this fanfiction. For example, an evil scientist who is jealous of Bulma's intelligence and wants to steal her inventions. This new villain could team up with other existing villains in the Dragon Ball universe to create chaos and try to break up Goku and Bulma's marriage.