Well, for me, it could be the first encounter. When the two characters meet in a really unexpected or funny way, like bumping into each other and then starting to argue but there's a spark. Another memorable moment might be when one of them makes a grand, stupid gesture to win the other's heart, like showing up at their work with a big sign saying 'I love you'. And of course, the moment when they finally realize they are meant to be together despite all the crazy things that happened before.
A simple Nezuko and Zenitsu fanfiction could be that they go on a journey together. Along the way, they encounter other demon slayers who are initially skeptical of Nezuko. But Zenitsu stands up for her, explaining that she's different. This shows the depth of their friendship and how Zenitsu has come to trust Nezuko completely.
One common theme is self - discovery. Since Zenitsu has a complex personality, many fanfics focus on him finding out more about himself, his powers, and his place in the world. Another theme is friendship. He often forms strong bonds with other characters in fanfics, like Tanjiro and Inosuke.