A powerful vampire could be the main antagonist. Vampires and werewolves are often enemies in such fictional settings. This vampire could be after Akakuro's pack's territory or have a personal vendetta against Akakuro himself.
It could be another werewolf from a rival pack. This rival werewolf might be jealous of Akakuro's status or abilities within his own pack. He could start a feud that escalates into a full - blown conflict for the entire werewolf community.
A hunter could be the main antagonist. Hunters are often seen as a threat in stories involving wild or unusual creatures. They might be after the werewolf for its pelt or just because they see it as a dangerous beast.
Another plot could be that Akakuro is a werewolf with a special ability, like the power to see the future. He uses this ability to try and prevent a great disaster from befalling his werewolf brethren, but in doing so, he makes some powerful enemies within the werewolf community.
It could be the werewolf who bit Harry. Maybe the werewolf has some ulterior motives, like using Harry as a pawn in a bigger plan against the wizarding world. Or it could be some pure - blood wizard supremacists who see Harry's new werewolf status as a sign that he is now 'impure' and want to get rid of him.
It could be a dark wizard who wants to use the living werewolf hide for some dark magic. Maybe he wants to create an army of werewolves under his control.
It could be a powerful vampire lord who sees the hybrid as an abomination and wants to destroy them. This vampire lord might view the mixing of werewolf and vampire blood as a threat to the purity of the vampire race.
The main antagonist could be an evil werewolf who wants to use Harry's new - found status for his own dark purposes. This werewolf could try to convince Harry to embrace the more savage side of being a werewolf and turn against his friends and the wizarding world. He might also be in league with Voldemort or other dark forces, using Harry as a pawn in a larger, more sinister plan.
Perhaps a group of anti - technology extremists could be the main antagonists in an Ultron fanfic. They see Ultron as the symbol of all that is wrong with technology and are determined to take him down, no matter the cost. They might use their own advanced, but more rudimentary, technology to fight against Ultron. This would create an interesting contrast between Ultron's high - tech abilities and their more basic, yet effective, methods.
It could be a rogue ninja who wants to capture the female werewolf for her unique powers. He might see her as a tool to gain more power in the ninja world.
A rival medicine cat from another Clan might be the main antagonist. They could see Alderheart as a threat to their own status or the well - being of their Clan. So they might use dark magic or some underhanded tactics to bring about his death.
Vilgax could be a great antagonist. He could hack into the gaming systems to try and defeat Ben in a virtual world where the rules are different. He might use his advanced technology to create digital versions of his minions to attack Ben.