He could meet centaurs. Centaurs are known to live in the Forbidden Forest in the Harry Potter universe, and if Harry is living in the woods, it's very likely he'd cross paths with them. They might teach him about astronomy and prophecy in a more in - depth way than he learned at Hogwarts.
He could meet Hagrid. Hagrid often goes to Diagon Alley, so it's possible he'll run into Harry while he's shopping and give him some useful advice about what to buy or not to buy.
He could meet some of the Hogwarts ghosts. They might have some interesting stories or information about the wards that could help Harry in his task.
He might meet other ninjas who have gone rogue or are in hiding for various reasons. These could be ninjas who have had a falling out with their clans or the village leadership.
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 encounter magical creatures that are native to the woods. For example, he could come across a family of bowtruckles, those small tree - guardian creatures. He'd have to learn to survive on his own, perhaps finding ways to build shelters and forage for food. Maybe he'd discover a hidden magical spring in the woods that has special healing properties.
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.