Another concept could be a redemption arc for Nappa. He could be given a second chance in the fanfiction, perhaps by being saved by the good guys and then slowly learning about the value of life. He might start to protect the Earth instead of trying to destroy it. This would be a big change from his original role in Dragon Ball.
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.