One interesting aspect of Dragon Ball Z Cell fanfics could be exploring his origin in more depth. Maybe there's a story where his creation was different, not by Dr. Gero but by some other mad scientist. This could lead to a whole new set of powers and abilities for Cell.
A battle between a Kryptonian and Goku in his Super Saiyan form would be extremely exciting. Goku's ki - based attacks and speed as a Super Saiyan would be a great contrast to the Kryptonian's natural powers. The Kryptonian's heat vision could be pitted against Goku's Kamehameha wave. It would be a clash of two very different power sources and fighting styles.
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.
Fanfiction also allows for different endings to their stories. Instead of their fates in the original series, they could be redeemed in some fanfictions. Picture Frieza actually learning to care about others and using his power for good. It's a different take on their characters that fanfiction can offer.
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.
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.