One interesting Grand Priest fanfiction could be one where he has a secret past that is slowly revealed. Maybe he was once a regular Saiyan who was chosen by the gods for a special purpose. This could lead to a whole new perspective on his character and his actions in the Dragon Ball universe.
There is a fanfiction that explores the Grand Priest's origin. It details how he came to be in his position of power. It involves a lot of lore from different parts of the Dragon Ball universe and ties them together in a really interesting way. It's a long read but well worth it if you're interested in the character.
It could be another powerful being from outside the known Dragon Ball universe. Maybe a being from a parallel dimension who sees the Grand Priest as a threat to his own plans for universal domination. Another option could be a rogue Angel who has turned against the Grand Priest and the current order of things in the multiverse.
One idea could be exploring the Grand Priest's past. Maybe he had a different role in the universe long ago. Another could be a story where the Grand Priest has to make a difficult decision that goes against the will of the Omni - King. Or perhaps a story where the Grand Priest trains a new character to become a powerful being in the Dragon Ball universe.
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.