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 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.
One possible fanfiction could be where Harry discovers his ability to turn into a Black Lion Animagus during a dangerous encounter in the Forbidden Forest. He uses this form to protect his friends from a dark creature that has invaded the school grounds.
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.
I'm not entirely sure if there is a specific fanfiction with 'Harry is a black lion animagus' but it's a very interesting concept. The Harry Potter fandom is vast and diverse. Some fanfictions are really well - hidden. You might have to dig deep into the archives of fanfiction websites or even contact some of the more dedicated Harry Potter fanfiction writers to see if they are aware of such a work.
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.
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.
One plot idea could be that Harry and Ginny discover they are Animagus during their time at Hogwarts and use their forms to secretly explore the Forbidden Forest. Their Animagus forms might give them an advantage in finding hidden magical creatures or ancient magic that could help in the fight against Voldemort.