Dragon Ball Z is both anime and manga. The anime was adapted from the manga and they share the same story and characters, but presented in different mediums.
One alternative is to focus on character backstories. You could write about the early days of Piccolo, how he came to be the way he is. Another option is to do crossovers with other non - sexual universes. Maybe a story where the Dragon Ball Z characters meet characters from a magical anime world and they have to work together to solve a problem. Also, you can write about the relationships between the characters in a platonic way, like the deepening of the friendship between Gohan and Krillin.
A Kryptonian in the Dragon Ball Z universe could potentially be a very powerful addition. Their natural abilities like super strength, flight, and heat vision could make them a force to be reckoned with. However, they would also face challenges as the Dragon Ball Z world has its own power scale with ki-based powers. They might have to adapt to the different fighting styles and power sources. Maybe they could learn to use ki in addition to their Kryptonian powers, or they could find a way to combine the two in unique ways.
If Goku was a Sekirei, it would be really interesting. His Saiyan instincts for fighting and getting stronger would combine with the Sekirei powers. He would probably be one of the most powerful Sekirei, constantly seeking out strong opponents to battle and improve. His positive attitude would also likely change the dynamic of the Sekirei Plan, as he wouldn't be interested in the more cutthroat aspects of it.
Vegeta could be paired with Ike. Both are strong - willed and prideful characters. They would initially clash due to their similar personalities, but as they face a powerful enemy together, they would come to respect each other. Vegeta's Saiyan power and Ike's swordsmanship would make for an interesting combination in battles. They could also have some great banter as they compare their respective power levels and combat philosophies.
Goku could still be the main character even in a dimension - hopping fanfic. His adaptability and thirst for stronger opponents would make him a great fit to explore different dimensions.
Yes, there are likely fanfics that focus on character crossovers. Fans often enjoy imagining how characters from 'Dragon Ball Z' and '7 Deadly Sins' would interact. For example, Goku could meet Meliodas and they might have a friendly sparring match. The different power systems and personalities would create interesting scenarios.
The main character could be a Saiyan gamer who was born on Earth but discovers his Saiyan heritage through the game. He then uses his knowledge of the game to train faster and become stronger than he ever thought possible, kind of like Gohan but with a more gamer - like approach to his training.