He could meet an old wizard who has been living in Kings Row for centuries and has a vast knowledge of ancient magic.
Perhaps he'll meet a magical creature that is native to Kings Row and has the ability to communicate with him. This creature could guide him through the new environment and introduce him to other local inhabitants. There could also be a local wizard who is an expert in a type of magic that Harry has never encountered before, and they could have an exchange of magical knowledge.
There's a chance he meets a group of nomadic magic - users. These are people who travel across the universe in search of new sources of magic. They could introduce Harry to different forms of magic that are not based on the traditional elements like fire, water, earth, and air that he's used to on Earth. Instead, they might use things like starlight or cosmic energy as a basis for their spells.
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.
Maybe he'll encounter a group of young wizards and witches who are also patients. They could form a bond and share their magical experiences. For example, they might talk about their different encounters with magical creatures outside of Hogwarts. This group could also play a role in helping Harry solve a problem that arises during his time in the hospital, like a malfunctioning magical device that is causing chaos in one of the wards.
Hermione might meet a famous wizarding historian in a fanfiction. They could exchange knowledge about the history of magic, and Hermione could learn some long - lost spells or secrets. This would enhance her already vast knowledge and might help in future adventures.
He might meet Canadian witches and wizards. There could be a friendly wizard who shows him around the Canadian wizarding community.
He might meet international aurors. These are highly skilled wizards from different countries who are experts in dark wizard catching. They could teach Harry some new combat spells.
He could meet Fleur Delacour's family. Since Fleur is from France, her family might be an important part of the French wizarding community that Harry interacts with.
He could meet his future children, if he has any. They would likely be powerful wizards or witches with their own adventures. Also, he might run into the future versions of Hermione and Ron. They would probably be in important positions in the wizarding world, like Hermione might be a headmistress at Hogwarts or something.
He might meet the future versions of his friends like Hermione and Ron. Their children or grandchildren could be there, and they might have different personalities and magical skills compared to their predecessors.
He might run into some of his old professors like McGonagall. She would be glad to see Harry back at Hogwarts. And perhaps he'll also meet some new students who look up to him as a hero. These new students could ask Harry for advice on how to be brave in the face of danger and how to master difficult magic spells.