There are no such inappropriate 'Straight Shota' stories in the official Dragon Ball Z canon. Dragon Ball Z mainly focuses on epic battles, power-ups, and the adventures of Goku and his friends to protect the Earth from various villains.
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.
Shallot can be developed by giving him a backstory. For example, explain where he came from and what his early life was like. This could shape his personality and motives in the fanfiction.
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.