It could be someone who is jealous of their relationship. Maybe another character who has a crush on Mori and doesn't like Haruhi's sister getting close to him.
Perhaps an over - protective family member of Haruhi's sister who thinks Mori isn't a good match for her. This family member could try to keep them apart by creating misunderstandings or getting in the way of their meetings.
Well, it could be about their unexpected friendship at first. Maybe Haruhi's sister has some trouble at school and Mori, being the calm and reliable one, helps her out. This could gradually develop into a deeper bond as they discover they have more in common than they thought.
A common theme could be friendship development. Since Mori is often seen as a rather stoic character, the fanfiction might focus on how Haruhi's sister breaks through his reserved exterior and they form a unique bond.
Some 'Haruhi sister x Mori' fanfiction might explore their relationship in a more platonic way. For example, they could be on an adventure together within the world of the 'Haruhi' series. The fanfic might focus on how they interact with other characters in the group as well, and how their relationship affects the overall dynamic of the group.
Mori Senpai would likely become more openly affectionate. In the series, he was often reserved, but as Haruhi's husband, he would show his love more freely. Haruhi, on the other hand, would probably become more aware of Mori's needs. For example, she might notice when he's tired or stressed and take steps to comfort him.
Morgana could be a great antagonist. She might see Merlin's sister as a threat to her own plans for Camelot. Maybe she discovers that Merlin's sister has a power that could undermine her magic or her influence over the court.
One possible plot could be their daily married life. Mori Senpai might continue to be his calm and protective self, while Haruhi could be the more energetic and independent partner. They could face challenges like balancing their different social circles. For example, Mori's traditional family expectations and Haruhi's friends from the club. They might also have to deal with financial matters, like how to manage their household budget with Mori's more reserved approach to money and Haruhi's sometimes spontaneous spending habits.
It could be a rogue vampire who has a vendetta against the Cullens and sees Bella's little sister as an easy target to get to them.
One popular Mori x Haruhi fanfic could be 'A Day in the Life'. It focuses on their interactions during a normal day at school in the Ouran High School Host Club setting. It shows how Mori's quiet nature contrasts with Haruhi's more down - to - earth and sometimes sassy personality.
It could be a rogue Death Eater who survived Voldemort's fall and blames Harry and his sister for the downfall of their cause. This Death Eater might be more cunning and powerful than the others, and he targets Harry's sister specifically because she represents a new threat to their dark plans. He might try to use her as a pawn to draw Harry out and then destroy them both.
It could be an ancient monster that has a grudge against the family of Poseidon. Since Percy and his twin are children of Poseidon, this monster might target them specifically. Maybe it's a sea serpent that was once defeated by Poseidon and now wants revenge by going after his descendants.