One interesting story could be about the characters' reactions to a new time - traveling villain. In this fanfiction, the Hargreeves siblings might initially be divided on how to handle the threat, leading to some intense in - family drama.
First, really understand the characters. Know their personalities, backstories, and how they typically react in the show. For example, Luther is often the more serious and duty - bound one, so his reactions would be different from say, Klaus's more carefree nature.