Well, it depends on the specific fanfiction. Some fanfiction authors might create a really elaborate backstory where Harry Potter is somehow related to Merlin as an heir. They could base it on things like similar magical abilities, a prophetic connection, or a hidden bloodline. However, not all fanfictions follow this concept. There are many different takes on Harry's story in the fanfiction world, and this is just one of the more imaginative ones.
Another possible candidate in fanfictions could be Neville Longbottom. He has a deep connection to the magical world and shows great courage and growth throughout the series. His latent magical abilities, which are finally fully realized, could be interpreted as him being an heir of Merlin in a way. He has the potential to be a great wizard, just like Merlin was.
It could be a previously unknown student at Hogwarts. Maybe a shy Ravenclaw who has always been good at spells that require great intelligence and wisdom, which are traits often associated with Merlin.
Yes, there are quite a few. Some focus on how the character as the heir of Merlin uses ancient magic in the Harry Potter universe. You can find them on fanfiction platforms like FanFiction.net or Archive of Our Own.
Another power could be the ability to communicate with magical creatures on a deeper level. Since Merlin was said to have a great understanding of all things magical, the heir in fanfiction might be able to gain the trust and cooperation of creatures like centaurs and unicorns more easily than others.
One interesting plotline could be the discovery of Harry's connection to Merlin and how it affects his magic in ways not explored in the original series. For example, he might have access to ancient spells or a deeper understanding of magic's origin.
Power and responsibility is a big one. Since the character has a connection to Merlin, they have to deal with the power that comes with it and the responsibility to use it well. Also, the exploration of ancient magic and how it fits into the existing Harry Potter magic world is quite common. For instance, they might explore how Merlin's magic is different from what is currently taught at Hogwarts. There could also be themes related to prophecy and destiny, as the character's new identity as the Heir of Merlin may be tied to some greater purpose in the wizarding world.
Obviously, Harry Potter is a main character as the heir of Merlin. Hermione Granger is often included as she helps Harry figure out the implications of his new status and the mysteries that come with it. Ron Weasley might also be there, providing his usual loyalty and friendship.
Well, a cool plotline might be the conflict it creates within the wizarding world. Since Merlin and Morgana have such complex and often opposing legacies in the lore. Harry, as their heir, could be caught in the middle of different factions. One side might see him as a great hope for a new era of magic, while the other could view him as a threat to the existing power balance. Also, his relationship with the Ministry of Magic could change drastically as they try to either control or use his newfound connection to Merlin and Morgana.
Hermione and Ron are likely to be main characters too. They are Harry's closest friends in the original series, and in this fanfiction, they would probably play important roles in helping Harry understand and deal with his new identity as Merlin's heir. They might assist him in researching about Merlin, or they could be by his side when he faces new challenges because of this heritage.