There could also be other characters like Hermione. She might play a role in helping Harry and Neville with their potions research. For example, she could be the one finding old potions books in the library that contain the secrets they need. And Ron might be there too, providing some comical relief and also being a guinea pig for their new potions sometimes.
Obviously, Harry and Neville are the main characters. Harry, the Boy Who Lived, is always full of courage and a thirst for knowledge. Neville, on the other hand, has his own growth from a shy and clumsy boy to a confident and knowledgeable one.
One interesting plotline could be Neville, who is already good at Herbology, discovers a new way to use plants in potions with Harry's help. This changes the way potions are made at Hogwarts.
Obviously, Harry is a main character. Hermione might also be there as she is good at potions and can help Harry. And Snape could be a prominent figure as he is the potions teacher.
Luna Lovegood could also be an interesting addition. Her unique perspective on magic and the wizarding world might bring new ideas to the group. For instance, she could have insights about some of the more mysterious magical creatures or phenomena that they come across, which could be crucial in their quests or battles.
Harry Potter is likely to be one of the main characters. He and Neville often interact in the original series, and in the fanfic, their relationship could be further developed. Hermione Granger could also be a main character. Her intelligence and knowledge of magic could play a role in whatever adventures or situations Neville gets into. Ron Weasley might be there too, adding his humor and loyalty to the group.
Hermione is often a common character. She might be helping Harry with research related to the potions professor's strange behavior or new potions they are working on. For example, she could be scouring the library for information about a rare ingredient the professor has introduced.
Obviously, Harry Potter himself is the main character. He is the one who is teaching himself potions.
In some fanfictions, Voldemort or his remaining Death Eaters could still be the main antagonists. They might target Harry and Neville specifically because they see them as a threat to their plans of taking over the wizarding world again.
Hermione would probably be involved. She's always been there for Harry in the original series, so in this fanfiction, she might be the one trying to find a way to reverse the situation or help Harry through the process. She's smart with potions and magic, so she'd be a key character.
An old and reclusive potions master who has a secret knowledge about Snitches and potions could be a main character. He or she could be drawn out of seclusion when some strange events start to occur involving Snitches and potions. This character would bring a lot of mystery and depth to the fanfiction, as they have a vast knowledge of the magical world that others don't.
Bellatrix Lestrange could also be a great antagonist. She could target their alliance as she is loyal to Voldemort. She might try to capture them to use as bait for other Order of the Phoenix members or to simply gain information about their plans to resist the Dark Lord.