Goku is always popular. He's the main character in the original series and in Battle of Gods, so in fanfiction, he often takes center stage in new adventures and battles.
Often, fanfictions in this area explore the concept of the gods' true nature more deeply. In the original Battle of Gods, we only got a glimpse. But in fanfiction, authors might expand on what it really means to be a god in the Dragon Ball Z universe. They could explore things like how the gods' power is maintained, their relationships with each other beyond what we saw in the movie, and how mortals can truly interact with them on a deeper level.
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.
In many fanfictions, Gohan's battle with Cell is often depicted with intensified emotions. Gohan might show even more of his hidden power, perhaps unlocking new forms or techniques that were not in the original series. This can make the fight more exciting and unpredictable.
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.