A new character could be introduced as the main character. Maybe a transfer student from a far - off magical school who already has some knowledge of Phoenix Slayer magic. This character could bring a fresh perspective to the Hogwarts environment and help the familiar characters in their fight against the various threats in the wizarding world.
Hermione could also be a great main character. With her thirst for knowledge, she could be the one to stumble upon the secrets of Phoenix Slayer magic. She would then use her intelligence to master it and perhaps teach it to her friends to help in their battles.
A Muggle - born blacksmith who discovers the wizarding world and uses his Muggle skills combined with magic he learns to create unique magical items. He could be the main character, struggling to fit in the wizarding world while also making a name for himself as a great magic blacksmith.
A young orphan who was raised by a hermit in the mountains and discovers they have the mountain dragon slayer magic. They are initially naive but grow into a powerful slayer through their adventures.
One interesting plot idea could be a crossover where the Phoenix Slayer magic is introduced into the Harry Potter world and a character has to learn this new form of magic to defeat a dark wizard. Maybe the Phoenix Slayer magic can control the power of phoenixes in a different way than what is known in the original Potterverse.
It could be an apprentice of the ancient potter. This character would be on a journey to learn the ancient potter's magic and secrets.
It could be a dark wizard who wants to control the holy dragon for his own evil purposes. He might see the dragon as a weapon to take over the wizarding world.
A powerful dark wizard who wants to steal the Phoenix Slayer's power for himself could be a great antagonist. He could have an army of dark creatures at his command and constantly be devising plans to defeat the Phoenix Slayer and take over the magical world.
Hermione could be a great main character in an elementalist fanfiction. Her intelligence and thirst for knowledge would make her eager to learn about and master elemental magic. She could be the one to discover a long - lost elemental spell book and start exploring the powers.
It could be a new student at Hogwarts who has a natural talent for dealing with beasts. This student could be from a family that has always had an affinity for magical creatures but has kept it a secret until now.
A Muggle - born mouse. It could be a unique concept where a mouse from the Muggle world somehow ends up in the wizarding world. It would be interesting to see how it copes with all the magic and the rules of the wizarding society.
Neville could also be a main character in this type of fanfiction. He could stumble upon a clockwork - related mystery in the Hogwarts gardens. Maybe there are some clockwork - powered plants that are acting strange. Neville, with his new - found confidence, takes it upon himself to solve the mystery and in the process, uncovers a deeper secret about Hogwarts and its connection to clockwork magic.