It could also be a character from outside of Hogwarts. Maybe a young wizard who has been self - taught in some dark magic and Dumbledore takes them as an apprentice to guide them towards the light side of magic. Dumbledore would see the potential for good in this character and want to reform them.
Another plot could be the apprentice getting involved in a time - traveling adventure. They might travel back in time to witness key events in the wizarding world's history, like the founding of Hogwarts. Along the way, they could accidentally change something in the past, and then have to find a way to fix it before it causes chaos in the present.
Ron could also be there. He might be a bit jealous at first of Harry's special relationship with Dumbledore but then comes around and supports Harry in his adventures as an apprentice. There could be some great scenes of their friendship evolving in this new context.
Another option could be Luna Lovegood. She has a very unique perspective on magic already. If she were Merlin's apprentice, she could bring her eccentric ideas and combine them with Merlin's ancient magic. Her friendship with Harry Potter and the others could also play a big role in how she uses her newfound status as Merlin's apprentice to help the wizarding world.
It could be an original character created just for the fanfic. Maybe a young wizard or witch who has a special talent for alchemy and catches Flamel's eye.
Harry may also develop a greater sense of responsibility. Knowing that Dumbledore has chosen him as an apprentice implies a certain level of trust and importance. He would feel the need to live up to Dumbledore's expectations and use his new - found knowledge and skills to protect the wizarding world. He could start to take on more leadership roles and be more proactive in the fight against the dark forces.
Perhaps Neville Longbottom could take on this role. He has shown great courage throughout the series. His experiences with loss, like the death of his parents, could lead him down a path where he becomes the apprentice of death, using his newfound powers to protect others and bring a new balance to the wizarding world.
In some fanfictions, Harry as Dumbledore's apprentice might have to deal with a dark prophecy that only he, with Dumbledore's guidance, can solve. This could lead to a journey where he has to make difficult choices between different paths, each with its own consequences for the wizarding community. It would be a great exploration of Harry's character growth and his relationship with Dumbledore in a more in - depth and unique context.
His allies could include Hermione and Ron as always. Since they are his best friends, they would support him in his apprenticeship. Hermione might even help him study and research with Flitwick. Neville could also be an ally as he is also interested in learning more magic. As for rivals, Draco might be jealous of Harry's new position and try to undermine him. Some of the Slytherins who don't like Harry might also be his rivals in this scenario.
Obviously, Harry Potter and Severus Snape are the main characters. Harry is the young wizard who is becoming Severus's apprentice.
Dumbledore would be an excellent choice. He is extremely powerful and wise. His apprentice would learn not only about powerful spells but also about the greater good and moral responsibilities that come with magic.