Characterization of Nappa and Vegeta in fanfiction can vary greatly. Vegeta is frequently portrayed as having a sense of honor, although it's a Saiyan - style honor that might not always align with what others consider moral. He is also shown as having a bit of a temper. Nappa, in contrast, is sometimes shown as more carefree in a way. He just wants to fight and prove his strength, not really thinking too much about the bigger picture like Vegeta often does.
In Dragon Ball Z fanfiction, Nappa and Vegeta have a complex relationship. Nappa is Vegeta's subordinate. Vegeta is often portrayed as the more dominant and powerful one. He bosses Nappa around. Nappa, on the other hand, is loyal to Vegeta, at least initially. Their relationship can be explored in fanfiction in many ways, such as how they interact during battles or how their Saiyan pride affects their bond.
Vegeta would also be a major character in these interactions. Their Saiyan bond could lead to different scenarios. Maybe Vegeta would be initially angry that Nappa survived but then come around to the idea of having him as an ally again. It could also be a source of conflict as they might have different ideas on how to handle certain situations.
One possible plot for this fanfiction could be that Bardock's return is not a simple one. He might be brought back by some sort of time - travel or resurrection magic. His return could stir up old rivalries among the Saiyans. And as he tries to fit into the current Dragon Ball Z universe, he discovers a conspiracy that threatens not only the Saiyans but the entire universe. So, he has to fight against this conspiracy while also trying to reconnect with his son Goku in a more meaningful way.
Power - up themes are common. Writers often explore how Nappa could become even more powerful, perhaps through new training methods or by unlocking hidden Saiyan abilities. It gives the character a new level of badassery.
Obviously, Bardock himself. And of course, Goku would likely be in it since he's Bardock's son. Vegeta might also show up, as he is an important Saiyan character. There could also be other Saiyans like Nappa or Raditz, especially if the story is about Bardock trying to reform or reunite the Saiyan race.
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 Nappa's backstory before joining Vegeta. Maybe he had a family on Planet Vegeta that he lost, which could add depth to his character. For example, in some fanfictions, it's shown that he was once a kind - hearted Saiyan before the destruction of his planet turned him into a more brutal warrior.