One popular one might be those that explore their relationship in a modern high - school AU. It could show how they first realize their feelings and then share that tender kiss.
There are probably fanfictions where they kiss during an adventure. Maybe they're on a quest together and in a moment of great emotion or relief, they kiss. This type of story often builds up the tension before that moment. For example, they face many dangerous situations and rely on each other. As they overcome the final obstacle, the kiss is like a celebration of their bond and survival.
The popularity might be due to the unique pairing. Craig and Tweek have very distinct personalities in the original work. Their relationship is full of potential for interesting stories. When they kiss, it's a big moment that fans love to explore in different ways.
One great story could be 'A Moment of Clarity'. In it, Craig and Tweek's kiss is a turning point in their relationship. It beautifully captures their unique dynamic, with Craig's calmness and Tweek's nervous energy. Another might be 'The Unexpected Kiss', where the kiss occurs in a moment of high tension, perhaps during an argument that suddenly shifts into a moment of deep connection.
One popular theme is their relationship development. Fans often write about how they gradually fall in love and overcome obstacles in their relationship, like dealing with the reactions of their friends or family in South Park.
A very popular trope is the 'love triangle' variation. Sometimes there might be another character who seems interested in either Tweek or Craig, which adds drama. The 'roommate' trope is also seen where they end up living together. And the 'secret relationship' trope, where they have to keep their relationship hidden from others for some reason.
You can try Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and there are likely many stories about Tweek and Craig that include a kiss scene. Just use the search function and type in relevant keywords.
One popular story could be about their adventures in school together. Maybe they team up to solve a mystery in South Park Elementary, like the strange disappearance of a class pet. This kind of fanfic often plays on their unique personalities, with Tweek's nervous energy and Craig's laid - back but tough attitude.
Another common theme is adventure. Writers like to take the two characters on different adventures in South Park or even outside the town. It adds an element of excitement and exploration to the stories.
One popular trope is the 'opposites attract' scenario. Given that Craig is calm and Tweek is jittery, this makes for an interesting dynamic. Another is the 'protective Craig' trope where Craig looks out for Tweek. For example, in some fanfictions, when other kids pick on Tweek, Craig steps in.
One common theme is the exploration of their first love. Since they are young characters, many fanfictions delve into how they come to terms with their feelings for each other. Another theme is friendship - turned - love, as they start off as friends or at least acquaintances in the original work. Also, there are often themes of overcoming insecurities together, as both characters have their own quirks and vulnerabilities.
Well, I'm not that well - versed in specific 'Craig and Tweek pregnant fanfiction'. However, when looking for such stories, you could also check out some South Park fan communities. Members there might be able to point you towards some interesting reads. They often share and discuss fanfictions, and you might get lucky and find some great 'Craig and Tweek' ones with the pregnancy theme.