One possibility could be centaurs. Centaurs are intelligent and have their own code of ethics. They could raise Harry with a deep understanding of nature and the stars, which would give him a different kind of wisdom.
In this fanfiction, the 'Father of Monsters' might be an ancient wizard who has been in a deep slumber for centuries. He was awakened by some disturbance in the wizarding world and, feeling angry about the changes that have occurred, decides to create monsters to bring back what he believes was the 'true' order of the wizarding world. His knowledge of old magic gives him the ability to create extremely powerful and dangerous monsters that no one has ever seen before.
The werewolves are quite popular. Remus Lupin's story has a lot of potential for further exploration in fanfiction. Writers can create more werewolf characters or expand on the werewolf community.
Monsters can also be used to explore different aspects of the wizarding world's magic. Their unique abilities can lead to new magical theories or explanations in fanfiction. For example, how the magic used to control a certain monster might be related to an ancient and forgotten branch of magic.
One possible plot could be that a character, perhaps an unknown wizard, is revealed to be the 'Father of Monsters' in the Harry Potter universe. He could be creating new and dangerous creatures secretly, and Harry and his friends stumble upon this mystery during one of their adventures. Maybe these monsters are being used to disrupt the wizarding world's peace, and it's up to Harry to stop this 'Father of Monsters' and his evil plans.
Monsters can be a major plot device in Harry Potter fanfic. They can change the relationships between characters. For example, if a monster attacks a group of students, it can make them bond together more tightly as they fight for survival. Or, if a character has a special connection to a monster, like being able to communicate with it, this can lead to a whole new sub - plot. The monster might hold a key to a mystery, like the location of a powerful magical object, and the characters' quest to get that information from the monster becomes a central part of the plot.
Well, you could start with the idea that the Duel Monsters exist in a hidden part of the wizarding world. Wizards have been using them secretly for centuries, but it's not common knowledge. Then, a young wizard stumbles upon this secret and starts to explore it, integrating Duel Monsters into his regular duels at Hogwarts.
Voldemort could still be the main antagonist. He might see Harry's connection to the monsters as a threat to his own power and try to either use it for himself or destroy Harry to prevent him from uniting the monsters against him.
Well, a prevalent theme is adaptation. Harry has to adapt to the new rules and mechanics of Duel Monsters. This often involves a learning curve which can be an interesting part of the story. Also, discovery is a big theme. Discovering new monsters, new strategies, and how it all fits into his understanding of magic. Additionally, there's the theme of power. How the power of Duel Monsters compares to the power he has in his own wizarding world and how he can use it to his advantage or face challenges because of it. There are many more possible themes depending on the creativity of the author.
I know there are some fanfictions in this vein. When looking for them, it's important to keep in mind that different authors will have different takes on how Harry Potter would interact with Duel Monsters. Some might have him be a natural at the game, using his magical skills from Hogwarts to his advantage. Others could make it a struggle for him to adapt to this new type of magic. Reading reviews from other readers can also be helpful in finding the better-written ones.