Obviously, Harry and Snape are important. Then there could be Snape's mother. Maybe she has some secrets or a past that is intertwined with the wizarding world's dark history. Another important character could be Snape's father, who might have had an influence on Snape's upbringing.
Well, perhaps Harry stumbles upon Snape's family in an unexpected place like an old family manor. Maybe he discovers Snape has a long - lost sister who has powers similar to Snape's but has been hidden away for some reason.
Obviously, Harry and Hermione themselves are the central characters. Their spouses, if they have any in the fanfiction, would also be important. Their children would play a big role too, as they would be part of the new generation of the family. And of course, old friends like Ron, Ginny, and other Hogwarts alumni might make an appearance.
Will's family members are also important. His parents, for example, would be key characters. They are the ones who will form an opinion of Nico. Siblings, if Will has any, could also be important. They might be curious about Nico, or they could be a bit protective of Will and initially be a bit wary of Nico. There could also be other relatives that are part of the family dynamic in the fanfiction.
In addition to Harry, Ron, and Hermione, Neville could play a significant role. He has grown into a brave and respected figure. Also, Luna Lovegood with her unique perspective might be crucial. The professors at Hogwarts, like Professor McGonagall, could also be important. She might be in a difficult position, having to balance between supporting Harry and following the school and wizarding regulations.
His wife could be a former Hogwarts student he met during his own school days. She might be from a different house and they fell in love secretly.
Luna Lovegood might be in the fanfiction. She has a unique perspective on magic, and she could offer some interesting insights during the examination. Neville Longbottom could also be present. He's been through a lot in the wizarding world, and his presence could add another layer to the story. Additionally, some fanfictions might include characters from Snape's past, like Lily Potter. Her presence, either in flashbacks or in some magical form, could greatly affect the dynamic between Snape and Harry during the examination.
Regarding the other teachers at Hogwarts, some might be more lenient with him because of Snape, while others might be more strict, trying to make sure he doesn't get any special treatment. His relationship with Neville could also be unique. Neville, who had his own experiences with Snape, might initially be wary of the son. But as they get to know each other, they could find common ground in their struggles and in their attempts to break free from the shadows of their fathers' reputations.
Dumbledore could be involved. He often has his fingers in many pies at Hogwarts, so he might be the one who arranges for Snape to deliver the letter to Harry for some greater good or part of his grand plan. Another possible character could be Hermione. She might notice something strange about Snape delivering the letter and start to investigate, suspecting that there's more to it than meets the eye.
One major challenge would be the public perception. Snape has a rather dark reputation in the wizarding world, and Harry is a hero. People might not understand or accept their relationship, let alone them having a family. Their family could face discrimination or be the subject of rumors.
It could be Bellatrix Lestrange. She was always a fanatic follower of Voldemort and hated those who she thought were against the Dark Lord. Since Snape was a double - agent, she might have discovered his true loyalties and decided to kill him in the fanfic.