Friendship rekindlings would be a big part. There could be characters who had a falling - out in primary school and now see the reunion as a chance to make up. They might have a heart - to - heart conversation and realize that their old grudges were silly. And of course, there would be new relationships forming too, as some people who didn't interact much in primary school find that they have a lot in common now.
Perhaps the fanfic could revolve around Harry's adventures in the gifted primary school. He could discover a secret room in the school where there are ancient books that hold the key to some sort of mystery. He then teams up with his classmates to solve it, using their combined gifted abilities.
One idea could be Harry starting at a new primary school where he discovers a hidden magical garden on the school grounds. It's a place only he can enter because of his special wizarding abilities, and he makes friends with magical creatures there.
Obviously, Harry would be one of the main characters. Then there could be his best friend, let's say Tom. Tom might be a science whiz who helps Harry with his projects. There could also be the strict but kind teacher, Mrs. Smith, who encourages Harry to use his gifts for good.
By adding unique characters. For example, a new teacher who is actually a wizard in disguise but has lost most of his powers. He senses Harry's magic and tries to guide him in a very clumsy way. This adds an element of mystery and humor.
It could also be a bittersweet reunion. Some might have lost touch with their best friends from primary school and would be trying to reconnect. There could be a sense of nostalgia as they walk around the old school building (if the reunion is held there), remembering the corridors they used to run in and the classrooms where they learned their first lessons.
One possible plotline could be the young wizards and witches in primary school having their first encounters with magic outside of their homes. For example, they might discover a hidden magical creature on the school grounds and try to keep it a secret from the teachers while also trying to figure out how to take care of it.
Obviously, Harry Potter himself would probably be there. Hermione Granger would also likely make an appearance. Ron Weasley would surely come along with his typical sense of humor. Neville Longbottom might show up, now a much more confident wizard. Luna Lovegood could be there too, with her unique perspective on things. And perhaps some of their less - known classmates from primary school would also be present, like Dean Thomas or Seamus Finnigan.
Well, they could re - establish old friendships that had faded over time. Maybe someone has a big announcement, like getting married or having a baby. There might also be a bit of rivalry resurfacing, especially if they compare their achievements in the wizarding world. They could also talk about how different their lives are from what they expected when they were at primary school. For example, a person who thought they would be a great Auror ended up being a Healer instead.