Perhaps in Animagus form, Harry is mistaken for a common magical creature by the Dursleys and is treated even worse. But this also means he has more freedom to explore the neighborhood unseen, and he discovers some hidden magical secrets there. He could then use this knowledge to his advantage when he enters Hogwarts.
One idea could be that Harry, in his Animagus form from a young age, has a unique bond with the magical creatures in the Forbidden Forest. They protect him and he learns their ways, which gives him an edge when it comes to dealing with dark magic later on.
A possible plot could be that Harry's Animagus form is that of a bird. As a bird, he overhears many important conversations among the Death Eaters and other dark wizards. This early access to information helps him and his friends to plan their defenses and counter - attacks much more effectively. He can also fly away quickly from dangerous situations, which saves his life on multiple occasions.
One idea could be Harry discovers he's an Animagus early on, perhaps in his second year. He turns into a stag like his father and uses this form to explore the Forbidden Forest secretly. Another could be that he has to hide his Animagus form from the Ministry which is on high alert for unregistered Animagi.
A plot could be that a Gryffindor Animagus accidentally reveals their form during a Quidditch match. This causes chaos as everyone tries to figure out who it is. It also draws the attention of the Ministry of Magic who have strict rules about unregistered Animagi. The character then has to deal with the consequences while also trying to prove they mean no harm.
Another interesting plot could be that Harry's transformation into a Black Lion is a sign that he has a deeper connection to Salazar Slytherin than anyone thought. It turns out that Slytherin had an affinity for lions in a different way than Gryffindors. This discovery causes a stir in the wizarding world, and Harry has to deal with both the prejudice and the new opportunities that come with this revelation. He could use his newfound connection to Slytherin's legacy to bring together the houses in a way that has never been done before in the fight against the Dark Lord.
A cool plot idea could be that there is a dark wizard who is trying to steal Wakanda's vibranium using magic. Harry Potter and Black Panther, as Animagus, team up to stop him. Harry could use his invisibility cloak in his Animagus form for stealthy attacks, while Black Panther uses his enhanced strength and agility in his panther form.
One interesting plot could be how Harry's magical abilities are perceived in a world of nations. Maybe he uses his magic to solve international disputes. Another idea is that different nations try to influence his upbringing, each adding their own cultural and magical elements to his training.
One idea could be that Harry grows up with cat - like reflexes and stealth. He might be able to move around Hogwarts unnoticed more easily, like a cat prowling in the shadows.
In the fanfiction, it could be interesting if Harry uses his magic to protect the other orphans from bullies. He might not fully understand his powers yet, but his instincts kick in. And then when he finally gets his Hogwarts letter, the other orphans are both sad to see him go but also excited for him, and he has to figure out how to leave the only home he's known while also being eager to enter the wizarding world.
One interesting idea could be a character who was always thought to be a normal human but is actually a reverse animagus, meaning they turn into a human from an animal form instead of the other way around. For example, a cat that can transform into a wizard. It adds a whole new layer of mystery and backstory to the character.
Well, an interesting idea could be an Animagus talent that is passed down in a family, but with a twist. For instance, the Weasley family has a unique Animagus form that is different for each member, but they all share some common traits. Like Ron could be a clumsy dog Animagus, always getting into trouble in his dog form but with a big heart. It could add a family - centric aspect to the fanfiction.
In 'harry potter animagus mating fanfiction', there might be ideas about how different animagus species interact in a mating context. A cat animagus and a dog animagus might have a relationship full of playfulness and some natural predator - prey instincts that add an interesting layer. And perhaps the transformation process itself plays a role in their mating. Maybe the energy required for transformation is somehow related to the energy of their relationship, and when they are in love, the transformation becomes easier or has different effects.