Maybe a powerful demon from the Inuyasha world who senses Penny's unique energy (since she's from the Harry Potter world) and wants to consume it to become even more powerful. This demon would see Harry Potter and Inuyasha as threats to his plan and would constantly try to get rid of them while also trying to capture Penny.
It could be Voldemort from the Harry Potter world who somehow gains the power to enter the Inuyasha universe and wants to use Penny for some dark magic purpose. He sees her as a tool to achieve his ultimate goal of ruling both worlds.
Well, it could be a really wild and creative plot. Maybe Harry Potter somehow gets transported into the world of Inuyasha and meets Penny there. He could use his magic skills while also learning from Inuyasha's combat and survival skills. And Penny might be in some sort of danger that they both have to work together to save her from, like a dark wizard who has also found his way into that strange mixed - up world.
Perhaps a long - lost relative of the Potter family who feels they were unjustly excluded from inheriting the manor. This relative could be jealous of Harry's status and the wealth associated with the manor. They might use dark magic to try and claim what they think is rightfully theirs, leading to a battle of magic and loyalty within the Potter family and among Harry's friends.
Yes, there might be some out there. Fanfiction often combines different universes, and it's possible that someone has created a story where characters from Harry Potter exist in the Inuyasha universe or vice versa. You can try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net.
Maybe there could be a new antagonist, a dark shapeshifter who is jealous of Harry's power. This antagonist could be an old wizard who has been in hiding for centuries, waiting for the right time to emerge and take over the wizarding world. He sees Harry as a threat to his plans and tries to eliminate him.
Another possibility is a corrupted member of the Ministry of Magic. They might be trying to use Harry as a scapegoat to gain more power within the Ministry. They could be working with the Death Eaters or acting on their own, but they would be a significant obstacle for Harry as he tries to stay alive and free while on the run.
Perhaps a new dark wizard or witch who has infiltrated the Ministry of Magic and made sure Harry was sent to Azkaban. This antagonist could be working towards a bigger plan to take over the wizarding world and sees Harry as a major threat even when he's locked up.
It could be Voldemort. He might see Harry's new waterbending ability as a threat and try to find a way to counter it. Maybe he would seek out dark magic related to fire to oppose Harry's water powers.
It could be a new dark wizard who has risen to power. This wizard might see Harry as a threat to his plans, so he manipulates the situation to get Harry exiled.
Another possibility is a corrupted magical creature that has been influenced by dark magic from the 'sociates'. This creature could be intelligent and manipulative, causing chaos among the students at Hogwarts and the wizards in the 'sociates'. It would be difficult to defeat as it has unique abilities that are not well - known to either group.
Perhaps a group of dark wizards who think that by disrupting time, they can bring back the dark ages of wizardry. They target the Timekeeper as the only one who can stop them. They use dark spells and curses to try and stop the Timekeeper from fixing the time - related problems they create.