One idea could be exploring the relationship between the new character, the Demons Child, and the existing Hogwarts students. Maybe the child has unique powers that both intrigue and frighten others.
One idea could be a story where a young wizard from the Harry Potter world accidentally travels to the Bleach universe. He could discover his newfound spiritual powers and try to find a way back home while getting involved in the battles against Hollows. Another plot could be a crossover where the Hogwarts houses are somehow related to the different Soul Reaper squads in Bleach. For example, Gryffindors could have an affinity with the more combat - oriented squads.
One idea could be that Harry discovers his connection to the demons due to an ancient family secret. Maybe his ancestors made a pact long ago. Another could be that he has to use his new 'King of Demons' powers to fight a dark wizard who is trying to control the demon realm for evil purposes. He could also face challenges within the wizarding world as they fear his newfound powers.
One idea could be that Harry discovers he has a unique magical ability that even Dumbledore doesn't know about. This ability could be related to his connection with the very essence of magic. For example, he might be able to communicate with magical creatures in a way no one else can, which leads him to uncover secrets about Hogwarts' past.
Some fanfictions might focus on the relationship between Albus and Scorpius in more depth. Maybe they discover a hidden magical bond between them that gives them unique powers, and they have to keep it a secret from everyone else while trying to use it to solve a mystery in Hogwarts that the teachers can't seem to crack.
One idea could be that the second child discovers a long - forgotten magic spell that has the power to reverse time briefly. This could lead to some interesting adventures where they try to correct past mistakes in the wizarding world. Another idea is that the second child has a special connection with magical creatures that no one else does, and they use this connection to solve a mystery involving a dangerous creature that is threatening Hogwarts.
One idea could be exploring the alternate timelines more deeply. For example, what if Voldemort had actually won in one of those timelines? How would the wizarding world look like? Another could be delving into the relationship between Albus and Scorpius in a more complex way, like their adventures outside of Hogwarts that test their friendship.
One idea could be that characters from Fma use their alchemical skills in the wizarding world of Harry Potter. For example, Edward could try to transmute a magical object using his knowledge of equivalent exchange. Another plot idea might be that the Philosophers Stone from Harry Potter has some connection to the alchemy in Fma, and characters from both universes have to work together to protect it from dark forces. Also, perhaps a character from Fma gets sorted into one of the Hogwarts houses and has to adapt to the magical school life while still having their own alchemical powers.
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.