One common theme is power. Combining the magical powers from Harry Potter and the physical and combat - related powers of Saiyajin. Another theme could be about identity. How a character from one universe adjusts to the rules and culture of the other. For instance, a Saiyajin in Hogwarts trying to understand magic while still maintaining their Saiyajin nature.
Well, it depends on what you consider interesting. There are some 'Harry Potter Saiyajin fanfiction' out there. Some might focus on a crossover where a Saiyajin character ends up in the Harry Potter universe, perhaps with their super strength and unique abilities causing all sorts of chaos or helping in the fight against the dark forces. Others could be more about a Harry Potter character somehow gaining Saiyajin powers. However, the quality can vary a great deal from one story to another.
Well, Vegeta would be a likely character. He might be initially suspicious of the female Goku. Maybe he thinks she's a threat or just can't believe there's another Goku - like being. He could be there to add some comical moments as he tries to understand the situation.
In general, it's a bit of a toss-up. The strength of Manga Goku and Anime Goku can vary depending on the specific arcs and battles. Sometimes, it might seem like one is stronger in a particular situation, but that could change in another.
It's hard to say for certain. 'El Candidato' could draw inspiration from real-life situations, but it might also incorporate fictional elements to make it more engaging. You'd have to look into the background and production details to know for sure.