One common theme is the exploration of their relationship's development. It might show how they gradually realize their feelings for each other before the kiss. Another theme could be dealing with the reactions of their friends and family in the fictional world.
Their relationship could also face challenges. Maybe the other members of the group start to feel left out, which causes some tension. But Eleven and Mike work through it together, showing that their relationship is strong enough to handle external pressures. They could become more protective of each other, too, especially when it comes to facing the supernatural threats in Hawkins.
The popularity also lies in the fact that fans can create their own versions of how and why the kiss happens. It could be in a dreamy, romantic setting or a tense, dramatic moment. Each fanfiction offers a unique take on their relationship, and that's what makes it so appealing.
Fanfiction writers usually paint the first kiss between Mike and Eleven as a very emotional and significant moment. They may describe the light, like how the sun is setting or the moon is shining, creating a romantic backdrop. There could be a focus on their breathing, becoming more rapid as they get closer. And the actual kiss is often described as a moment of pure connection, where the world around them seems to fade away.
One great fanfiction could be 'The Unspoken Bond' where their kiss is a culmination of a long - hidden mutual attraction. It delves deep into their emotions and the context leading up to that magical moment.
Perhaps in the fanfiction, Motorcity is under a new threat that requires Mike and Julie to go undercover together. During their time in disguise, they have to pretend to be a couple. As they get into their roles more and more, the line between pretense and reality blurs, and they end up sharing a kiss. This kiss then makes them question their true relationship status.
Well, in many fanfictions about Mike and Amanda's kiss in Supah Ninjas, it could be a turning point in their relationship. It might be a moment of realization of their deep feelings for each other. Maybe it's a passionate kiss that comes after a dangerous adventure where they both thought they might lose each other. This kind of scene can add a lot of romantic and emotional depth to the story.
You can try searching on popular fanfiction websites like Archive of Our Own (AO3). It has a vast collection of fanfictions, and you can use the search function to specifically look for ones about Mike and Will kissing.
One idea could be that they share a kiss during a moment of great danger. Maybe they are facing a powerful enemy and think it might be their last chance to express their love. So, in the heat of the moment, they kiss passionately, giving them the strength to fight back.