Perhaps it's a magical crystal that enhances his ability to communicate with magical creatures. This would be quite unique as it would give him an edge in dealing with things like thestrals or unicorns, and open up new storylines in the fanfiction. Harry could use this crystal to understand their needs and gain their help in various situations.
One common theme is discovery. Harry discovers his magic in unexpected ways, like finding a hidden magical object or meeting a mysterious figure. Another theme is secrecy. Since he's not supposed to use magic outside of Hogwarts yet, he has to keep his newfound skills a secret from the Dursleys.
Besides Harry, there could be a character who is a descendant of an ancient elemental magic family. This character might initially be a rival to Harry in mastering the magic but later becomes an ally. Also, Dumbledore could play an important role. He might be secretly guiding Harry in his journey of learning elemental magic, using his own deep knowledge of the wizarding world's secrets.
There's a fanfiction called 'Early Magic for Harry'. In this story, Harry stumbles upon an old magic book in the cupboard under the stairs. This book helps him start learning magic even before he gets the Hogwarts letter. He begins with simple levitation spells and gradually progresses. It's a really interesting take on how Harry could have been more prepared for Hogwarts if he had known some magic beforehand.
He becomes more confident. With the unique magic focus, he has something that sets him apart. This newfound power or ability makes him less self - doubting and more assertive in his actions.
One great plot development could be that the elemental magic Harry learns is not part of the traditional Hogwarts curriculum. So, he has to seek out hidden mentors or ancient texts. This could lead to him exploring parts of the wizarding world that are long forgotten. As he delves deeper into elemental magic, he might also start to question the limitations of the magic he has been taught so far at Hogwarts.
One interesting plotline could be Harry discovering a new and ancient form of magic that was lost for centuries. This could lead him on a journey to understand its origin and how to master it.
It could be Dumbledore, even if he's passed away. His portrait or some kind of magical remnant of him might still be able to guide Harry in learning new magic.
In a lot of fanfiction, Harry knowing ancient magic is a really cool concept. It allows for all kinds of interesting plot developments. He could use ancient magic to fight against new and more powerful enemies. Maybe he uses it to unlock secrets about the wizarding world's past that were long buried. It also gives the author a chance to create their own unique magic system within the framework of the Harry Potter universe, and often Harry is at the center of this exploration of ancient magic.