One common theme is the survival aspect. Just like in regular zombie stories, the characters in the Harry Potter world have to find ways to stay alive. Another theme is the corruption of magic. Sometimes, the zombies are created by dark magic gone wrong.
Yes, there are some interesting 'Harry Potter Zombies' fanfictions. You can try searching on platforms like Fanfiction.net or Archive of Our Own. Some fanfictions might combine the magic of Harry Potter world with the horror and chaos of a zombie apocalypse in really creative ways.
Harry Potter himself is often a central character. Fans like to write about him using his Gryffindor courage to face the zombie threat. His ability to think on his feet and use magic in creative ways makes him a natural protagonist in these stories.
One idea could be that the zombies are created by a dark magic spell gone wrong in Hogwarts. Maybe a student accidentally unleashes it during a potions experiment. The students and teachers then have to use both magic and their wits to fight the zombies while also trying to find a cure.
Well, it could be that the zombies are magically created in the wizarding world. Maybe Voldemort's followers use dark magic to raise an army of zombies. Harry would use his spells like Expelliarmus to disarm the zombies first. Then he might use the Patronus charm to keep the zombies at bay as the Patronus can act as a powerful shield against dark creatures.
In such a fanfiction, perhaps Harry could use his magic to either command the zombies for some greater good, like using them to fight against Voldemort's forces. He might discover a new spell that allows him to control their minds. Maybe he first encounters the zombies in the Forbidden Forest and has to quickly figure out how to manage them before they cause chaos in Hogwarts.
It could be a dark wizard who also wants to control the zombies but for evil purposes. Maybe a long - lost relative of Voldemort who has emerged from the shadows and sees the zombies as a means to finally take over the wizarding world. He might try to steal Harry's method of controlling them or create more powerful zombies that are immune to Harry's control.
One interesting crossover could be the use of spells in the Call of Duty Zombies setting. For example, a wizard character from Harry Potter might use the 'Stupefy' spell to stun zombies instead of using a regular weapon. Another could be the Hogwarts castle being overrun by zombies, and the students and teachers having to fight them off using a combination of magic and the survival skills from Call of Duty. It could also be cool to see the Death Eaters team up with the zombies for some sort of dark alliance.
Yes, there are quite a few. Some fanfics combine the magical world of Harry Potter with the horror of zombies in really creative ways. For example, there might be a story where Hogwarts is under a zombie invasion and the students have to use their magic to fight off the undead while also dealing with the usual Hogwarts politics and rivalries.