I think magical creatures would also be customers. Maybe house - elves would be sent to pick up treats for their wizarding families. Hippogriffs might be lured by some special bird - shaped pastries. And even centaurs, if they were feeling a bit more adventurous, could come to the edge of the Forbidden Forest where the bakery might be located to get some goodies.
Definitely Hogwarts students. They'd come during their free time or on trips to Hogsmeade. They'd be eager to try Harry's special magical treats.
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.
Ninjas, of course! They would come to relax after a long day of training or missions. Also, fans of the ninja world from within the story would be there. Maybe even some of the village leaders would stop by to show support for the local business.
Other ninjas would be likely customers. They need things like kunai, shuriken, and ninja wire which Naruto could sell in his store.
Her old Hogwarts friends would be very likely customers. They would come to support her new business and also to relive their old memories together. Hermione would probably come often to discuss books and new spells while having a drink.
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.
Voldemort would still be a very likely villain. He would see the Hulk as a threat or a tool to be used for his own evil purposes. He might try to control the Hulk's mind or use dark magic to make the Hulk do his bidding.
His Hogwarts friends would probably be the first. Hermione would be eager to check out any new books or intelligent - enhancing potions. Ron would be interested in any food - related magical items, like everlasting candies.
His old friends from Hogwarts would surely be regulars. Ron and Hermione would be there often. Neville might come to relax after a day of teaching Herbology at Hogwarts. Luna could drop by to share her latest theories about magical creatures while sipping on a strange - looking drink. Hagrid might also be a regular, though he'd need a special - sized chair.
Mostly wizards and witches, of course. They would be the main clientele as they are the ones who can fully appreciate the magical elements of the restaurant. Students from Hogwarts during Hogsmeade weekends or holidays would also flock there.