Maybe the pegasus is not just a regular familiar but is actually a cursed one. The curse could be that it can't fly during the day or something similar. The main character then has to find a way to break the curse while also dealing with the normal Hogwarts life and challenges. This would make for a very interesting and complex fanfiction.
A pegasus could be a time - traveling familiar. It could take the main character back in time to witness important events in the Harry Potter universe's history. This would add an exciting twist to the story.
The crow familiar could be a bridge between the Muggle and wizarding worlds in the fanfiction. It can interact with Muggle animals in a way that normal wizarding familiars can't. For example, it can communicate with crows in the Muggle park and learn things about the Muggle world that are useful to Harry. It could also have a connection to a magical constellation. When the stars align in a certain way, the crow gains extra powers, like the ability to fly faster or see through invisibility cloaks. And you could write a story where the crow's nest is a portal to a secret magical realm that only it and Harry can access.
In some fanfictions, he might find it in the Forbidden Forest during one of his adventures. Maybe while exploring the deeper parts of the forest that are off - limits to most students, he stumbles upon a young pegasus in need of help. Harry, being the kind - hearted person he is, bonds with the pegasus and it becomes his familiar.
In fanfiction, a Pegasus could be a symbol of great power or prophecy. It could be foretold that only a chosen wizard can tame a particular Pegasus, and this wizard will then be able to use the Pegasus's power to defeat a great evil that threatens the wizarding world. This would create an exciting plotline full of mystery and adventure.
Foxes could be part of a new magical species in the fanfiction. They could have their own language and culture within the wizarding world. For instance, they might live in a hidden part of the Forbidden Forest and have a special connection to nature magic that the main characters need to understand to defeat a new dark force. This would expand the world - building in the fanfiction.
One way is to have a Creepypasta - like curse befall Hogwarts. For instance, a curse that makes the staircases move in a truly chaotic and disorienting manner, similar to the unease that Creepypasta stories create. The students would have to navigate this new and scary situation, perhaps with some help from the teachers who are also baffled by this strange occurrence.
One unique way is to make the Tower of London a key part of a time - travel plot. Say, a powerful spell was cast in the Tower long ago that is now causing disruptions in the present - day wizarding world. Harry and his friends have to travel back in time to the Tower's past to stop the spell from being cast in the first place. The Tower could be filled with different eras of magic and different types of magical defenses that they have to overcome. Also, the Crown Jewels in the Tower could be magically enhanced items that have a special connection to the wizarding world, like they can be used to unlock a powerful magical vault.
Well, one ability could be enhanced speed. Pegasus are already very fast, but in the fanfiction, Harry's pegasus might be able to travel at near - instantaneous speeds. This would be useful for getting Harry out of dangerous situations quickly or for delivering urgent messages across long distances in the wizarding world.
One unique way is to make the dragon fire act as a form of communication. Imagine a dragon that can send messages through the patterns of its fire. In the fanfiction, the main character has to learn to decipher these messages to solve a mystery or prevent a disaster. Another idea is to have the dragon fire be a source of time - travel. When the fire engulfs a certain magical object, it can send the holder back in time, leading to all sorts of interesting plot twists in the Harry Potter universe.
You could make the heritage potion a key part of a mystery. Maybe it's the only thing that can solve a series of strange disappearances in the wizarding world. The characters have to track down the ingredients, which are scattered across different magical locations, and then brew the potion correctly. Along the way, they learn about different family heritages and how they might be connected to the mystery.
A succubus familiar can be incorporated by first establishing its magical nature. For example, it could have powers related to charm and allure, which are common traits of succubi. In a fanfiction, it might be a familiar that a dark wizard or an outcast wizard has. The succubus familiar could use its charm to help its master in various ways, like getting information from other characters or sneaking into restricted areas.