One idea could be to cross UI Goku with a character from another popular anime, like Naruto. UI Goku could end up in the Hidden Leaf Village and have to deal with the different ninja powers and politics there. Maybe he'd have to train some of the young ninjas, which would be really cool as his fighting style is so unique.
Bulma could be involved. Since she has a relationship with Vegeta and is friends with Goku, she might try to mediate between them when Goku snaps at Vegeta. She could be the voice of reason, trying to calm both of them down and make them see each other's point of view.
It's hard to say for sure. Both Goku UI and Jiren are incredibly powerful in the manga. Their strength and skills are on par, so it could go either way depending on various factors.
First, decide which character or universe you want to cross UI Goku with. This is crucial as it will set the tone for your whole story. Then, think about how Goku gets to this new world or meets the new character. It could be through some kind of interdimensional portal or a magical spell gone wrong.
It often starts with a shock or surprise at the mpreg situation. Then, they gradually learn to rely on each other more. For example, Vegeta might start to show a more caring side that he usually hides, and Goku might become more responsible. They'll have to communicate more, sharing their fears and hopes about the future with the baby.
Yes, in the vast world of fanfiction, there are all kinds of creative and out - of - the - box stories. Some fans like to explore unusual relationships and scenarios, so it's possible that there is such a fanfiction out there. However, it goes against the original canon of the Dragon Ball series in many ways.
Well, if they were brothers in a fanfic, it could change how they interact with the other characters. Maybe they'd have a deeper understanding and fewer power struggles between them. Also, their family background and upbringing could be explored in new and interesting ways.