Perhaps it's a hunter from a parallel universe. In his universe, Spiderman is a feared and hated figure, and this hunter has come to our universe to hunt down those who support Spiderman. He sees the fan as a symbol of the support he wants to eradicate.
The hunter could be Kraven the Hunter. He is known for his obsession with hunting the most dangerous prey, and in this case, he might see a Spiderman fan as an interesting target. Maybe he thinks the fan has some connection to Spiderman that he can exploit.
One possible plot could be that the hunter is a villain from Spiderman's rogues gallery who mistakes a die - hard Spiderman fan for someone important to Spiderman. Maybe he wants to use the fan as bait to lure Spiderman into a trap. Another plot could involve the hunter being from a different dimension or world and thinking the fan has some sort of power related to Spiderman, so he kidnaps the fan to study or harness that power.
The Phantom Troupe could be antagonists. They are a group of dangerous criminals in the Hunter x Hunter world, and they might see Kaito as a threat or a target for their own purposes. Another potential antagonist could be some of the more corrupt members of the Hunter Association who might not like Kaito's independent ways. There's also the possibility of new, original antagonists created specifically for the fanfiction, like a powerful dark magician who wants to use Kaito's power for his own evil plans.
It could be a new character created specifically for the fanfiction. This character might be an outcast from a powerful clan in the Hunter x Hunter world, with a grudge against the hunters, thus being named the 'white devil' due to their menacing and unpredictable nature.
It could be a rogue hunter who has his own twisted ideas about the use of Nen. This hunter might see Pokkle and Ponzu as obstacles in his quest for power and knowledge. He could try to capture them to steal their Nen abilities or the information they have about certain areas of the Hunter x Hunter world.
Another possibility is that Deadpool kidnaps Spiderman because he's bored. He sees Spiderman as this straight - laced hero and figures that by kidnapping him, he can mess with Spiderman's life a bit. He could take Spiderman to some really strange places, like a dimension full of dancing hot dogs (because it's Deadpool, anything is possible) and Spiderman has to find a way to escape while also dealing with Deadpool's crazy antics.
There could also be a corrupt official within the Hunter's Guild. This person doesn't like the way the Handler and Hunter are operating, perhaps because they are too independent or they are getting in the way of some underhanded scheme the official has. So, this official uses their power to try and undermine the duo, from withholding resources to sending them on extremely dangerous and near - impossible missions.
How about a character who is a master of disguise within the Hunter x Hunter world? They could have a Nen ability that allows them to not only change their physical appearance but also their aura and nen - type. This would make them very difficult to detect and could lead to some really sneaky and devious plotlines. For example, they could infiltrate enemy groups or steal valuable information without being noticed.
Gon and Killua would be obvious choices. They already have a great dynamic and their friendship could be further explored in a school setting. For example, they could be roommates and have to deal with each other's annoying habits while also training together.
The plot could also involve her having unique nen abilities different from Hisoka's. She might be on a journey to understand these powers and in the process, cross paths with the main characters from Hunter x Hunter. Perhaps she forms an unlikely alliance with Gon and Killua, who initially are wary of her because of her relation to Hisoka, but later come to respect her for her own qualities.
Hisoka is extremely popular in fanfiction. His unpredictable and somewhat insane nature makes for great plot - driving elements. Fanfictions might explore his motives more deeply or put him in situations where he has to interact with the main characters in unexpected ways. Additionally, characters like Illumi also gain popularity as his complex relationship with Killua can be explored in many different and fascinating ways in fan - created stories.