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.
Maybe he'll meet some of the shopkeepers in a more personal way. Not just as a customer, but they might recognize him as the famous Harry Potter. The owner of Weasleys' Wizard Wheezes, for instance, could give him some special new products to test out, seeing as he's friends with the Weasley family. And it's also possible that he meets a mysterious wizard who has some connection to his past or future, adding an element of mystery to his shopping trip.
There's a chance he'll meet some other Hogwarts students. For example, students from different houses might be there. He could meet a Slytherin student who might try to taunt him or a Hufflepuff student who could be really friendly and offer to share some shopping tips.
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.
Harry could meet some local magical creatures that are unique to Africa. For example, there could be a magical creature similar to a hippogriff but with different characteristics. And he may also meet African witches who are experts in herbal magic, which they use to heal and protect their communities. They could teach Harry some of their techniques and knowledge.
He could also meet some sort of dark wizard or villain that is even more powerful than Voldemort in this new world. This would pose a great threat to Harry and force him to use all of his skills and knowledge, both from his own world and what he's learned in the new one, to defeat the enemy.
He might meet other Hogwarts students who he doesn't know very well. They could become good friends through their shared love of Quidditch.
He might meet the house - elves that have been taking care of Potter Manor for generations. They could show him around the manor and tell him stories about his family.
In this fanfiction, Harry might meet students from various backgrounds. There could be students from far - off magical lands who have their own unique cultures and magic. He could also run into some Beubaxtons alumni who come back to visit and they might share some interesting stories or give him some advice on how to make the most of his time at Beubaxtons.
He could meet his future self. This would be a very interesting encounter as he could learn from his future self about what he has accomplished and what mistakes he should avoid. He might also meet the future generations of his family, like his grandchildren or great - grandchildren.
Perhaps Harry will run into an old wizard who has been in the hospital for a long time. This wizard has a lot of stories about the old days of the wizarding world and gives Harry some valuable advice.
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.
There could be a group of students who are known for their pranks in the fifth house. Harry would probably cross paths with them and might either get involved in their antics or try to stop them. Another possibility is that he meets a shy student who has some special abilities but is too afraid to show them. Harry could be the one to encourage this student to come out of their shell.