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.
Another option for the main antagonist could be a group of vampire hunters who mistake Shadow for a true, evil vampire. They constantly hunt him down, which forces Shadow to fight back. However, this also draws the attention of real vampires who see Shadow as a way to get revenge on the hunters, and Shadow has to deal with both threats.
Obviously, Shadow the Hedgehog is a main character. He's the one with the vampire - like qualities. Sonic could also be a main character in some fanfictions. He might be in conflict with Shadow because of his new vampire nature or could be trying to help him control it. Amy could be another main character, perhaps she has a special connection to Shadow and tries to understand his transformation.
One interesting plot could be Shadow the Hedgehog's struggle to control his new vampire - like powers. For example, he might have to deal with an insatiable thirst for blood while still trying to protect his friends and the world. Another plot could involve Shadow being hunted by a group of vampire hunters who mistake him for a true evil vampire, leading to a lot of action - packed chase scenes.
One idea could be that Shadow discovers he has vampire-like abilities after a strange encounter in a dark, enchanted forest. He then has to struggle with his new thirst for blood while still trying to protect his friends and the world from villains.
The main antagonist could be an evil sorcerer who has corrupted the Black Knight and is using him to carry out his dark plans. This sorcerer could be after Sonic's speed power to enhance his own magic.
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.
Obviously, Shadow himself would be a main character even in his baby form. Sonic would also be a key character as he is often the hero in these Sonic - related stories. He would likely be involved in trying to figure out how to turn Shadow back. Tails could be another main character as his intelligence would be useful in finding a solution to Shadow's transformation. He might study the cause of it and try to come up with a remedy.
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.
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 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.
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.