An alien race that has a long - standing feud with vampires could be the antagonist. They see the vampire traveling with the Doctor as a threat to their plans of galactic domination. They try to capture or kill the vampire, and the Doctor has to protect his friend while also finding a way to make peace between the two sides.
A powerful vampire lord could be the main antagonist. This vampire lord might want to use the Doctor's time - traveling abilities for his own dark purposes. Maybe he wants to spread vampirism across different time periods and planets. The Doctor and the vampire companion would have to stop him to prevent chaos in the universe.
Well, perhaps the 9th Doctor and the vampire could be on a journey to find a cure for the vampire's condition. They might travel through different time periods and encounter various historical figures who have knowledge about vampires. Along the way, the Doctor tries to teach the vampire about the value of all life forms while also facing challenges from other creatures who oppose their mission.
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.
Perhaps a half - alien, half - vampire hybrid. This character was shunned by both species and blames Ben for making him feel even more like an outcast. He uses his unique powers to create chaos and tries to turn Ben into a vampire - alien like himself.
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.
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 the main antagonist is a scientist who has been experimenting with vampire-like powers and accidentally creates a powerful vampire - like monster. This monster then starts attacking the world, and Shadow, as a new vampire - hedgehog, has to stop it. The scientist might also be trying to cover up his mistake and will stop at nothing to keep Shadow from interfering.
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.
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.