The Pandora Hearts universe itself provides a rich backdrop. The dark and fantastical world allows for all kinds of scenarios in the fanfics. Writers can play with the existing lore, add new elements, and create unique stories that still feel like they belong to the Pandora Hearts world. And for Vincent and Gil, their roles within this world give a lot of room for creative interpretations. For example, their different allegiances and how that affects their relationship can be explored in countless ways.
The complex relationship between Vincent and Gil. Their connection is full of mystery, loyalty, and a hint of something more. Fanfic writers can explore this in many ways, like delving into their past or imagining different futures for them.
The characters' personalities are also a big draw. Vincent's enigmatic nature and Gil's more straightforward but loyal personality create a great dynamic. Fanfic authors can focus on their individual traits, how they complement each other, or how they clash. There are so many emotions to explore - from friendship to something deeper like love or a sense of brotherhood that might be hidden beneath the surface.