One interesting fanfiction could be about a human character discovering a new form of power that is unique to humans, not Saiyan-based. For example, a story where a human martial artist figures out how to tap into the latent spiritual energy within all humans to achieve a power level that can rival the Saiyans.
Training is a key factor. In fanfictions, humans can train in extreme environments, like on a planet with high gravity. This forces their bodies to adapt and become stronger, increasing their power. Just like how Goku trained on King Kai's planet.
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.
One interesting concept could be the extreme power level inflation. In some fanfictions, characters' power levels reach astronomical numbers that were never seen in the original series. It makes for really epic battles. For example, a fanfic might have Goku reaching a power level that can destroy entire universes with a single punch, far beyond what was shown in the official Dragon Ball storylines.
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.