Megumi is not dead in the JJK manga. The manga continues to feature him and his adventures. The story hasn't reached a point where his character is killed off.
Often, they highlight their shared love for food. Since they are both in the culinary world in the original story, fanfics usually revolve around cooking challenges they face together or how they teach each other new techniques.
One common theme is the discovery of love. Takumi and Megumi often start as friends or acquaintances and gradually realize they have deeper feelings for each other. Another theme could be personal growth. Through their relationship, they both learn more about themselves and become better individuals.
Love and friendship are also big themes. It can start as a friendship that slowly blossoms into love. The journey of how their relationship evolves from just acquaintances to something deeper is often explored. Like in some fanfics, they start as classmates at Jujutsu High and then feelings start to develop over time.
One common theme is the contrast between their personalities. Sanosuke is often portrayed as more rough - edged and impulsive, while Megumi is more refined and composed. Their relationship then becomes a journey of how these differences attract them to each other. Another theme is protection. Sanosuke may be seen as the protector of Megumi, which adds a layer of romance and chivalry to the story.
There's a fanfiction called 'Unlikely Allies'. In this story, Kenshin and Megumi are forced to work together on a mission. It's full of action and also some really sweet moments between them as they get to know each other better. As the story progresses, you can see their relationship evolving in a very natural way.
One common theme is the development of their relationship against a backdrop of conflict. Since they are in a world full of danger, they often have to rely on each other, which deepens their bond.
Sukuna does die in the manga. The way it happens is quite dramatic and ties in with many other elements of the story, adding depth and tension to the overall narrative.