Perhaps he meets a mysterious stranger who has a deep knowledge of tennis and shares some interesting anecdotes with Harry. This stranger might even have a connection to the world of magic if it's a fanfiction with some magical elements.
In some fanfictions, Harry might meet a goblin with a special mission for him. This goblin could be an outcast among his kind and believes that Harry is the only one who can help. So, he takes Harry on a journey through the deepest parts of Gringotts, where Harry gets to see parts of the bank that no one else has ever seen. During this journey, Harry also meets other strange and interesting characters that are somehow connected to the goblin's mission.
He might meet some interesting characters there. Maybe he'll encounter a famous tennis player who gives him some tips on the sport.
He might meet his parents. Since they have passed away, the other side of the veil could be a place where they exist in some form.
There's a chance he'll meet other outcast wizards. These wizards might have been shunned by the wizarding world above ground. They could be very different from the wizards Harry is familiar with, perhaps having their own unique styles of magic. They might initially be hostile to Harry, seeing him as an outsider from the 'normal' wizarding world, but later they could become his allies. They might also know secrets about the underground world that could help Harry in his future battles or quests.
He could meet a young and brilliant wizard inventor. This inventor is working on some newfangled magical gadgets and shows Harry some prototypes, like a self - flying broomstick that can change its shape according to the rider's mood.
In the fanfiction, Harry could meet Native American wizards or witches who have a deep connection to the land and ancient magic. Their magic might be based on nature and spirits in a way that is very different from what Harry is used to in Britain. He may also meet some Muggle - born American wizards who have a different perspective on being part of the wizarding world, having grown up in a different cultural and social environment compared to Harry.
There's a chance he'll run into Ron. Maybe Ron is looking for some new wizarding sweets at Honeydukes, and they end up sharing some of the treats while they continue their shopping spree. And it's possible that they'll see Neville there too, who is searching for some rare plant seeds for his Herbology studies.
When Harry Potter goes to McKinley in fanfiction, he'd likely meet the jocks and the nerds alike. The jocks might be initially intimidated by his differentness, while the nerds would be fascinated and want to pick his brain about all things magic. He could also meet the school outcasts, who would find a kindred spirit in him.
Well, aside from Thor and Odin, he could meet Sif. Sif is a strong Asgardian warrior, and she might be interested in Harry's unique brand of magic. And of course, there's a high chance of meeting Loki. Loki is always up to something, and his devious nature could either make him an adversary or an unexpected ally to Harry.
He would most likely meet Peter Pan. Peter is the main character in Neverland and he would surely cross paths with Harry. Also, he might meet Tinker Bell, the feisty fairy. And of course, the Lost Boys would be there too, and they would probably be curious about this new boy with strange magical powers.