There could be students from a nearby school or college. They might come in for a quick snack or to study while having a coffee.
Hermione would probably be a regular. She'd come to the cafe to study while enjoying a warm cup of Butterbeer latte. She might even bring some books from the Hogwarts library to read there.
Well, first of all, the genin teams would be regulars. They need to stock up on basic ninja equipment. Then, the older ninjas who are retired might visit to relive their younger days by buying things they used to use. And don't forget the wandering ninjas. If Naruto's shop becomes famous enough, they would stop by to buy rare items or get some rest and information at the same time.
Well, the regular customers could include some of Harry's old friends from Hogwarts, like Ron and Hermione. They would support Harry's business and enjoy the unique atmosphere. In addition to that, supernatural hunters from the Supernatural world might stop by. They could be looking for information about certain supernatural beings that might be lurking around the bar. And of course, there would be the local supernatural community, such as witches and warlocks who are not directly related to Hogwarts but are interested in the mix of magic and the new - found supernatural elements in the bar.
In the 'Harry Runs a Cafe' fanfic, there could be a situation where Harry hires a new and very eccentric staff member. This person could bring in a whole new style to the cafe, like unique music or strange but delicious recipes. Another interesting event could be a local festival where Harry decides to set up a booth outside the cafe to attract more customers.
His ninja friends like Sasuke, Sakura, and Kakashi would surely be regulars. Sasuke might come to buy things secretly to support Naruto's business. Sakura could be there to get supplies for her medical ninja work if it's a general store. And Kakashi would stop by for a chat and to pick up some new reading material if Naruto sold books.
Fellow wizards or magic - users could be regulars. They might come to the bar to unwind after a long day of using magic. Some might be there to exchange stories about their magical adventures or to seek advice from Harry, who has his own share of extraordinary experiences. They could include a young wizard just starting out, looking for guidance from Harry, who is seen as a bit of a local legend in the bar.
He would probably encounter a lot of kids who are fans of his ninja skills. They would be excited to see Naruto up close and ask for autographs or for him to show off some small tricks. There would also be regular customers who just come for the good food and coffee, and they might be initially surprised to see a ninja working in a cafe but then get used to it.
I bet the regular customers would include some of Harry's old Hogwarts friends. Maybe Ron would stop by for a butterbeer and a chat. Hermione might come to relax after a long day of research. And of course, the Weasley twins would probably be there often, looking for new business ideas or just to have some fun.
Obviously, Harry is one of the main characters since he owns the cafe. There might also be his employees, like a barista who is really good at making coffee and has a great personality.
He could meet American wizards and witches. Maybe there's a young and enthusiastic wizard who becomes his guide in America, showing him around the American magical communities.