A plot idea could be that Ron discovers a memory in the Weasley family attic that shows a connection between the Weasleys and an ancient magical artifact. This discovery could lead to an adventure where the trio has to figure out the significance of the artifact and protect it from dark wizards who also want it for their own nefarious purposes.
You could write about a memory of Hermione's first time using magic as a little girl. It might show her natural talent and her eagerness to learn more. Then there could be a memory of Ron's family during a particularly difficult time, like when they were short on money but still managed to have a warm and loving home. And finally, a memory of the Marauders in their prime, causing all sorts of mischief at Hogwarts and how those memories still linger in the castle.
One idea could be an apprentice who discovers a long - lost spell that can bring inanimate objects to life in a very different way than the usual magic. For example, they make Hogwarts portraits come down and interact physically with students.
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 a revolt against the Ministry of Magic's unjust laws. Maybe the characters unite to fight against discriminatory regulations that target certain magical creatures or wizarding groups. Another could be a student revolt at Hogwarts against a particularly tyrannical headmaster or teacher. For example, if the new rules are too strict and limit students' freedom to learn magic in their own ways.
Well, a cool plot for a 'Harry Potter and Bellatrix Fanfic' might be that Bellatrix, in a moment of confusion during a battle, uses a spell that transports her and Harry to a different time period. There, they have to rely on each other to survive and in the process, Bellatrix starts to question her loyalty to Voldemort. Also, a plot where Bellatrix discovers that Harry has a hidden power that could be used to bring back someone she lost, and she bargains with him for it could be interesting.
Another plot idea might be that the wizarding world discovers Harry's changeling status and there is a big debate about whether he should be allowed to continue at Hogwarts. Some might fear his different nature, while others see it as an opportunity to learn more about the magical world's secrets.
Well, a cool plot might be that some students at Hogwarts start to develop a resistance to the Dementors' effects due to a new magical potion they accidentally create. Harry could be involved in helping them perfect this potion. Also, a story where Dementors are used as a weapon in a wizarding war but then start to turn on their masters. Or maybe Harry has a vision of his parents being attacked by Dementors and he has to go back in time to try and save them.
One idea could be that the Ministry of Magic discovers a connection between Atlantis and a powerful dark magic. Harry and his friends are sent on a mission to Atlantis to stop a dark wizard from awakening an ancient evil there. They'd have to navigate the unfamiliar Atlantean magic and the strange underwater environment.
One idea could be exploring the hidden secrets within the manor. Maybe there are long - forgotten rooms filled with ancient magic artifacts. Another could be a story where the characters hold a grand ball at the manor, but dark forces try to disrupt it.
One idea could be a fanfic where Neville's toad, Trevor, gains the ability to talk and becomes a sort of magical detective, helping solve mysteries around Hogwarts. Another could be a story about Hagrid finding a baby dragon that has a special bond with Harry's owl, Hedwig. They team up to protect the Forbidden Forest from dark wizards who want to steal the dragon's unique powers.