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.
Another possible villain could be Fenrir Greyback. He is a werewolf in the Harry Potter world, and Wolverine also has some werewolf - like qualities. Fenrir might see Wolverine as a rival. He could team up with some dark wizards to try and take down Wolverine and Harry Potter. This would create a complex web of battles and power struggles between different factions in the fanfiction.
Voldemort could still be the main villain. He might see Noah Puckerman as an oddity in his world and try to use him for some dark purpose or just get rid of him because he doesn't understand where he came from.
Voldemort could be a great villain. Imagine him getting his hands on some of Spider - Man's technology. He could use it to enhance his Death Eaters' powers or even create new, more powerful dark magic. Spider - Man and Harry would have to put aside their differences in fighting styles and worlds to stop him.
In a Harry Potter x Bella Swan fanfiction, the most likely villain could be a combination of characters from both universes. Fenrir Greyback from the Harry Potter world could team up with a rogue vampire from the Twilight world. Greyback's love for chaos and the vampire's thirst for blood could create a really dangerous duo. Another option is Bellatrix Lestrange. She might be jealous of Bella's relationship with Harry (if there is one in the fanfiction) and try to harm her. Plus, her loyalty to Voldemort could make her a major threat as she would carry out his evil plans regarding this new and strange arrival, Bella.
Loki could be an interesting villain in this crossover. Loki, being the trickster god, might see the wizarding world as a new playground for his mischief. He could manipulate some dark wizards into causing chaos, which would draw the attention of both Harry Potter and Nick Fury. Harry, with his knowledge of magic, and Fury, with his strategic mind and resources, would have to work together to outwit Loki and his minions.
Obviously, Harry Potter would be one of the main characters. He is the central figure in his own universe and would bring his magic and heroism to the crossover. The Hulk would also be a main character, with his super strength and dual - identity. His presence would add a lot of action and chaos to the story.
Another possibility is Winky. She is also a house - elf. Winky was a bit of a tragic character in the original books. In the fanfiction, if Harry were to summon a servant, her meek and somewhat lost nature might make for an interesting addition to Harry's side, and she could be summoned to help Harry with various tasks.
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.
Harry Potter himself could interact with Sophie from Huntik. Harry is used to facing great challenges and having a strong sense of justice. Sophie, with her connection to the Titans, could find common ground with Harry in their fight against evil. They could learn from each other's worlds and work together to overcome a powerful dark force that threatens both universes.
Definitely Harry Potter himself. He has faced so many dark and dangerous things already, so he'd be able to handle zombies. Hermione would also be a likely survivor. Her vast knowledge of magic and quick thinking would serve her well. And Neville Longbottom. He has shown great courage and has a deep understanding of Herbology, which could be useful in creating defenses or finding antidotes.