Well, it might be about the continuation of the relationship that started with a kiss. Maybe it shows how the characters face new challenges and how their love endures or changes in 'the last kiss' scenario.
Most likely the main characters would be the ones from the original 'it started with a kiss' series. They would be the ones whose relationship started with that kiss and are now facing 'the last kiss' situation.
I'm not entirely sure as I haven't read it in detail. But generally, it might be about a relationship that begins with a kiss and then unfolds various romantic and perhaps challenging events as the story progresses.
You could also focus on a side character's perspective. For instance, a friend of the main characters who has always secretly liked one of them. This character could try to interfere with the relationship in 'it started with a kiss' in the fanfiction, creating a lot of drama and tension in the story.
One common theme is also the exploration of relationships. How that one kiss affects their relationship, whether it's a new start or a complication in an existing relationship.
In a small village, Oscar is known as the kind-hearted boy. One day, a hedgehog enters his life. At first, the villagers are afraid of the hedgehog, but Oscar defends it. He takes care of the hedgehog and they develop a strong bond. One evening, as the sun sets and they are sitting by the pond, Oscar kisses the hedgehog, symbolizing their acceptance and love for one another.
One possible plot could be a post - war scenario. Draco, having redeemed himself to some extent, meets Astoria at a pure - blood gathering. Their initial encounter is filled with a bit of tension as they come from different yet similar backgrounds. As they start to talk more, they realize they have more in common than they thought. A slow - burning attraction develops, and finally, they share a kiss that symbolizes a new beginning for both of them, away from the dark pasts of their families.
It could be a story where they finally confess their feelings after a long time of being friends. Maybe they are in a dangerous situation together, and in that moment of adrenaline, they share a kiss.
Maybe in a story where Robin is investigating a strange magical occurrence and Zatanna is helping him. As they get closer to the truth, they start to develop feelings for each other. One day, after a particularly difficult discovery, Zatanna comforts Robin and they end up kissing. This kiss then changes the dynamic of their relationship as they continue to solve the mystery.
It could be that Leah is a shy person and Nahuel is the more outgoing one. They meet by chance in a park. As they start to know each other better, Nahuel slowly breaks down Leah's walls. The kiss is a culmination of their growing connection, and after that, they have to figure out how to build a relationship with their different personalities.
Maybe they are in a dangerous situation and Natsu saves Wendy. In the heat of the moment, they share a kiss out of relief and a new - found closeness.