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.
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.
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.
Well, if Harry Potter wields Excalibur in fanfiction, it would be a wild combination. Harry is already a powerful wizard, but Excalibur is a sword of legend. It might change the whole dynamic of the wizarding world. For example, the sword could cut through dark magic in a way his wand can't. Maybe it would attract the attention of other magical creatures who respect the sword's power. And it could also create some interesting relationships with Arthurian legends within the Harry Potter universe.
One idea could be that the Excalibur sword somehow finds its way into the wizarding world of Harry Potter. Maybe it's brought there by a powerful magical being from another dimension. The sword could possess unique magical properties that interact in strange ways with the existing magic in the Harry Potter universe, like being able to cut through any dark magic spell.
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.
Another plotline could involve the relationship between Harry and the wizarding world's reaction to his new status. Some might see him as a greater threat to Voldemort, while others might fear his new - found power. There could be a power struggle within Hogwarts as different factions vie for Harry's allegiance.
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.
To start a 'Harry Potter Excalibur' fanfiction, think about the rules of magic in both settings. In the Harry Potter world, magic is learned and controlled through spells and wands, while Excalibur has its own kind of magic. Decide how these magics will interact. For instance, will the sword enhance a wizard's spell - casting ability? Then, introduce the conflict. Maybe Voldemort wants to get his hands on Excalibur for its power. Build your story around this conflict and the characters' attempts to prevent it.
It could be a new dark wizard who is jealous of Harry's ability to lift Excalibur. This wizard might want the sword for himself to gain ultimate power. He could start causing chaos in the wizarding world, trying to draw Harry out and take the sword from him. He might even team up with some of the Death Eaters who are also interested in the power the sword could bring.