One great fanfiction could be where Mutsumi and Shinomiya go on a trip together. They might start off as friends but gradually realize their deeper feelings. It could include sweet moments like sharing an umbrella in the rain and stealing glances at each other.
Another trope is the 'forced proximity'. Maybe they have to share a small space for some reason, like being locked in a library or a storage room. This closeness makes them more aware of each other's presence and leads to the development of their relationship.
A plot idea could be that Igarashi and Kae get paired up for a dance competition. Kae is initially nervous as she's not very confident in her dancing skills, but Igarashi patiently teaches her. As they practice, they start to have feelings for each other that they didn't expect. They then have to deal with their new - found emotions while also preparing for the competition.
Igarashi and Kae's relationship is like a budding flower. Igarashi is like the sun, constantly shining his attention on Kae. He is always there for her, whether it's helping her with something or just being a good friend. Kae is starting to notice his efforts more and more. Their relationship is based on mutual respect and a growing attraction. There are moments of shyness and hesitation, but also of great warmth and connection.
The ending of 'Kiss Him Not Me' manga ties up most of the storylines neatly. The main character finds love and resolution in a way that's satisfying for fans.
Fanfiction often allows for creative crossovers. In 'kiss x sis crossover fanfiction', it could involve combining the world or characters of 'kiss x sis' with another franchise. Maybe it's to explore new relationships or storylines that couldn't exist in the original work. For example, if it's crossed with a magical world setting, the characters from 'kiss x sis' might have to adapt to new magic rules and face different challenges.