In a more unexpected twist, Merlin himself could show up. Since he's associated with Excalibur in the Arthurian legend, he could be the one who tests Harry to see if he's worthy of pulling the sword. Merlin could then teach Harry how to use the sword's power to its fullest extent, given his own deep understanding of magic.
Dumbledore could also play a role. Even if he's not alive in the normal sense, his spirit or some form of his magic could guide Harry. He might have known about the connection between Harry and Excalibur all along and left behind some hints or instructions for Harry to follow.
Well, it could be that in a magical crossover world, Harry Potter somehow stumbles upon the fabled Excalibur. Maybe he's on a quest to defeat a new dark wizard who can only be vanquished with Excalibur's power. So, he has to prove himself worthy to pull the sword, just like in the Arthurian legends.
Hermione could help. She's very smart and could research about the dragon's weaknesses.
Perhaps it's a story where Harry Potter is transported to a world that combines elements of the wizarding world and a modern, gun - using society. He pulls a gun to protect his friends when they are attacked by some unknown and powerful force that magic has no effect on. This could lead to him having to learn about the new rules and ethics of using such a weapon while still relying on his magical abilities.
Well, perhaps in the fanfiction, Harry Potter is on a quest outside of the wizarding world and stumbles upon Excalibur. The sword recognizes his pure heart and heroic nature, allowing him to lift it. This event could then draw the attention of both the wizarding and Muggle worlds. Harry would then have to deal with the new responsibilities that come with wielding such a powerful and legendary sword, all while still having to deal with the threat of Voldemort.
Obviously, Harry Potter is a main character as the heir of Excalibur. Hermione Granger would also likely be involved. She is always by Harry's side in the original series, helping him with research and providing intelligent solutions. So in this fanfiction, she might be the one to help Harry figure out the implications of being the heir.
His friends from Hogwarts, like Ron and Hermione, are likely to help. Ron could bring his family's practical knowledge of wizarding homes and Hermione would contribute with her extensive research skills.
Well, if Harry Potter finds Excalibur, it could completely change the wizarding world. He might gain some new powers from the sword. Maybe he could use it to defeat Voldemort in a more epic way than with his own wand. It could also bring a new alliance between the wizarding world and some other magical beings who respect the power of Excalibur.
Hermione could be a great help. She's very knowledgeable about magic and could research how to properly care for a newly hatched female dragon.
Hermione would be a great help. Her vast knowledge of magic and books could provide the necessary information for building the planet, like the right spells and magical theories.
Dumbledore might also play a role. He has vast knowledge and experience. He could offer some wise advice or even provide a special magical item or spell that is crucial for the unicorn's healing.