Well, perhaps there's a magical object that connects Harry to the Pendragon legacy. As he uncovers this connection, he gets drawn into a world of ancient magic and battles. He could also form alliances with characters from Pendragon stories to fight against a common evil that threatens both his wizarding world and the Pendragon realm.
In a 'Harry Potter is Lord Pendragon' fanfiction, a plotline could be that the wizarding world is on the brink of a new kind of danger. Harry, as Lord Pendragon, is the only one who can stop it. He has to journey to different magical places, some related to the Harry Potter universe and some from Pendragon lore. Along the way, he discovers more about his true identity and how to use his combined powers. He might also have to deal with jealousy or disbelief from his friends in the wizarding world as they struggle to understand his new role.
One possible plotline could be that Harry discovers he has a secret heritage as Lord Pendragon. This leads to him having new powers and responsibilities. He might have to face off against enemies from both the wizarding world and the Pendragon - related threats.
One possible plotline could be that Harry Potter discovers he has some sort of connection to Lord Cahill, like a long - lost family tie. This could lead to him having new powers or a different role in the wizarding world.
One possible plotline could be that Harry discovers he has a hidden connection to the Black family that makes him Lord Black. This could lead to him having new powers or responsibilities within the wizarding world, and he might have to deal with the complex politics and family secrets of the Blacks.
One common theme is the combination of magic systems. The magic from the Harry Potter universe and the supposed magic related to Lord Pendragon are often merged. Another theme is heritage. Characters usually discover their connection to the Pendragon line and how it affects their role in the wizarding world.
One interesting plotline could be the discovery of the connection between being the heir of Pendragon and Harry's existing powers in the wizarding world. Maybe it gives him new abilities or a different perspective on magic.
One plotline could be that Harry is influenced by the Dark Lord's ideology from an early age. Maybe he discovers some 'truths' as he sees them that make him believe the Dark Lord's way is the right one. For example, he might think that the wizarding world's current order is corrupt and only the Dark Lord can fix it.
Perhaps Harry is reincarnated as Lord Emrys in an alternate timeline. He then has to use his new - found identity and powers to reshape the wizarding world for the better. He might face challenges from those who oppose his new - found status and powers, while also trying to uncover the secrets of his past life as Lord Emrys.
In some fanfictions, the heir of Arthur Pendragon could have a special connection to the magical creatures in the Harry Potter universe. Since Arthur was a great king, the heir might inherit his ability to communicate or form bonds with these creatures. This connection could be crucial in solving various mysteries in the wizarding world, like finding a hidden place where a powerful dark magic object is hidden. They could use the help of these creatures to outwit the villains who are also after the same object.
Another plotline could be a time - travel scenario. Draco gets sent back in time to Camelot. He becomes an apprentice or a follower of Arthur Pendragon. He then has to learn the ways of Arthur's court, deal with the politics and magic there, and in the process, he grows as a person, bringing back some of Arthur's wisdom to his own time when he eventually returns.
One possible plotline could be a blind character in the 'Harry Potter' world who has a special ability related to their blindness. Maybe they can'see' magic in a different way than others, like being able to sense dark magic more acutely. In a 'Lord of the Rings' context, a blind character might have an enhanced sense of hearing or smell to compensate, and they could play a crucial role in the quest, like being able to detect approaching enemies from afar.
I've heard of a fanfiction titled 'Pendragon's Magic at Hogwarts'. It's really cool as it imagines how the Pendragon magic would interact with the magic of the Harry Potter universe. For example, the use of ancient Pendragon spells that are different from the normal spells in Hogwarts. The main character, who has Pendragon blood, has to learn to control both types of magic and use them to fight against dark forces that threaten both the wizarding world and the legacy of the Pendragon.