A powerful vampire lord could be the main antagonist. This vampire lord could be ruling over a hidden vampire society and sees Lupin as a threat to his power, especially if Lupin is starting to gain vampire - like abilities.
It could be a vampire scientist. This scientist is conducting cruel experiments on both humans and vampires to create a new breed of super - vampires. Lupin stumbles upon this operation during one of his heists and decides to put an end to it. The scientist then becomes obsessed with capturing Lupin and using his unique skills and knowledge for his experiments.
It could be a rogue Death Eater who escaped justice and is now seeking revenge on the Order and anyone related to them. So, Teddy, being Lupin's son, would be a target.
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 a dark wizard who is trying to prevent Teddy from meeting his father. This wizard might be afraid that Teddy will gain some powerful knowledge or magic from his father that could be used against him.
One idea could be that Lupin discovers he has a vampire ancestor and starts to develop some vampire - like abilities which he has to learn to control while on a heist. Another could be Lupin teaming up with a vampire hunter to steal a powerful vampire - related artifact from a group of evil vampires.
It could be a powerful vampire clan that doesn't want Harry, a new vampire, to disrupt their ways. They might try to either kill him or force him to join their ranks against his will. They could see him as an outsider who doesn't belong in the vampire world and a potential danger to their secrecy.
Perhaps a vampire scientist who is conducting unethical experiments on both humans and vampires. Ichigo gets in his way when he discovers what the scientist is doing. This antagonist could be using his scientific knowledge to create powerful and dangerous hybrids. He believes that he is creating a new and better vampire - human world order, but his methods are causing a lot of harm, and Ichigo is determined to stop him.
It could be an old vampire who sees Elena as a threat to his power or territory. Maybe he doesn't like the idea of a newly - turned vampire causing a stir in the vampire community and decides to take her out.
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.
A powerful elder vampire who opposes the prince's ideas of change within the vampire community. This elder might see the prince as a threat to the traditional vampire ways and will stop at nothing to undermine him.