Perhaps a powerful vampire lord who sees Sookie as a threat to his power. Since she was once human and has connections to both worlds, he might fear that she could rally other vampires and humans against him. He could try to eliminate her by any means necessary, leading to a lot of action - packed scenes in the fanfiction.
Another option is a human hunter who has made it his mission to kill all vampires, especially those who were once human like Sookie. He may think that Sookie is a more dangerous kind of vampire because she can better blend in with humans. So he would be constantly on her trail, creating a tense cat - and - mouse dynamic throughout the story.
It could be an old and powerful vampire who sees Hermione as a threat to his rule. Since Hermione is a very intelligent witch - now vampire - she might be able to figure out a way to undermine his power.
It could be an old and powerful vampire lord who sees Natsu as a threat to his rule. Since Natsu is a new and different kind of vampire, the lord might want to eliminate him.
Maybe Voldemort or his followers could still be antagonists. Even though Harry is now a vampire, Voldemort might want to use his new state for his own evil purposes or simply view Harry as a nuisance that must be destroyed. Since Harry has changed, he might have new weaknesses that Voldemort could exploit.
Another possibility is a vampire who wants to use Ethan for his own gain. He might try to manipulate Ethan into doing things that are against his will. This vampire could be part of a powerful vampire clan and wants Ethan to be his puppet so that he can gain more power within the clan.
Another option is a powerful vampire coven leader who wants to use Jacob for his own purposes. When Jacob refuses, the coven leader turns against him and becomes the main source of conflict in the fanfiction.
Well, one possibility is that Sookie becomes a vampire and uses her unique abilities as a former human with telepathic powers to bridge the gap between humans and vampires even more. She could start a movement to promote better understanding and co - existence, which would put her in a dangerous position as there are many vampires and humans who oppose such ideas.
The vampire Authority could be antagonists. They might see Sookie as an abomination since she was a fae - human before being turned into a vampire. They could try to eliminate her to maintain the 'purity' of the vampire race according to their own rules.
It could be another vampire clan. They might see Cartman and Kyle's relationship as a threat to their own power or way of life. For example, if Cartman and Kyle are trying to promote co - existence between vampires and humans, the other clan might oppose it.
It could be another ghost who is jealous of Casper's new human form. This ghost might try to sabotage Casper's attempts to fit into the human world, perhaps by causing chaos around him and making humans afraid of him again.
It could be a character who wants to use Percy - turned - Aku for their own evil purposes. Maybe a dark wizard who thinks that Aku - Percy can be the ultimate weapon to take over the world. This wizard would try to manipulate Percy's new form and powers to do his bidding.