In some 'harry potter apocalypse fanfic' plots, there could be an invasion from another dimension full of dark and powerful creatures. This invasion could be the start of the apocalypse. Harry Potter and his allies would have to quickly learn about these new threats and find ways to defend the wizarding world. It might also involve a situation where the Ministry of Magic fails completely during the apocalypse, and it's up to the students and a few rogue wizards to save the day. They could discover new and hidden magics in the process of fighting for survival.
One idea could be that the Forbidden Forest becomes the breeding ground for the zombies. The magical creatures there are either zombified or in a fight for survival. Hogwarts students, led by Harry Potter, have to use both magic and their wits to fend off the undead hordes while also trying to find a cure within the castle's ancient secrets.
Maybe the apocalypse was caused by a dark magic experiment gone wrong. Now, Harry Potter's friends and family are scattered, and he has to journey across a desolate wizarding world to find them and also figure out how to reverse the damage. Along the way, he discovers new forms of magic that emerged as a result of the chaos.
Maybe Voldemort somehow resurrects an army of zombie Death Eaters as a last - ditch effort to take over the wizarding world. But this time, the good side has to team up with some unexpected allies, like the giants and the house - elves, to defeat this new threat. The story could explore how different magical creatures react to the zombie apocalypse and how their unique abilities can be used in the fight.
One idea could be that the villains release a powerful virus that turns most of the population into mindless zombies. The heroes have to not only fight the villains but also deal with the zombie horde. Another could be an apocalyptic event that wipes out most of the world's quirks, and the remaining heroes and villains must adapt to this new powerless world.
Well, you could have a fanfic where Voldemort uses DND - style dark magic to open portals to other dimensions. The Order of the Phoenix has to quickly learn DND - style combat and magic rules to fight off the new threats that come through these portals. Harry could be sent on a special mission to find a DND - style mage who can teach them the counter - spells. Meanwhile, the Death Eaters are also trying to master this new form of magic for their own evil purposes. This would create a really exciting and unique story.
Perhaps a plot where the protagonist can choose to either side with the Ministry of Magic during a time of political unrest within the wizarding world or join a rebel group. This would involve different magical battles and moral dilemmas depending on the choice. In the 'harry potter cyoa fanfic', this could show different aspects of the wizarding society that we don't often see in the original books.
A cool plot for a Harry Potter Twilight fanfic could be a competition. The wizards from Harry Potter and the vampires and werewolves from Twilight compete in a series of challenges. These challenges could test both magic and the unique abilities of the Twilight characters. Or perhaps a love triangle between a Harry Potter character, a Twilight vampire, and a Twilight werewolf. It would be interesting to see how the different cultures and powers interact in matters of the heart.
Another plot could focus on the lives of the secondary characters from the original series. For example, Neville could be the head of Herbology at Hogwarts and discovers a new, dangerous plant that has some sort of connection to an ancient magic. The 'carry on fanfic' could then be about how he and his students deal with it, with some cameos from the main characters.
One idea could be that a dragon escapes from Gringotts and Harry has to use his skills to track it down. Maybe the dragon has a special connection to a powerful dark magic item that Voldemort was after.
One idea could be that Voldemort somehow enters the Runescape world to seek more power. Harry and his friends follow him there to stop him. They have to learn the rules of Runescape quickly to be able to fight Voldemort. Another idea is that a Runescape wizard discovers a portal to the Harry Potter world and brings some of the unique items from Runescape, like special runes, which causes a stir in the wizarding world.