The key element would be the build - up to the kiss. There needs to be a progression in their relationship in the fanfiction, whether it's through shared battles, emotional conversations, or just spending time together. Another element is the 'lemon' part which could involve some level of sensuality in their relationship after the kiss. It could be described through their body language, their thoughts, or the atmosphere around them.
In this fanfiction, the Natsu and Lucy kiss could be a dream - like sequence at first. Lucy might be dreaming about her feelings for Natsu, and in the dream, they kiss. However, the lemon aspect could be that this dream has a very sensual undertone. When she wakes up, she's left with these confusing and strong emotions that she then has to face Natsu with. It could lead to an interesting exploration of how they both really feel about each other and how they handle this new level of intimacy that the dream has introduced, even if it was just in her mind at first.
Yes. The kiss between Natsu and Lucy is a pivotal moment that adds depth to their relationship in the manga. It comes as a surprise to many but is a welcome development for their love story.
One common theme is the exploration of their unspoken feelings. Often, in the series, their relationship is somewhat ambiguous, and fanfiction allows for that to be explored more deeply with the kiss as a climax of those pent - up emotions.
Yes, there are quite a few. You can check out fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Lucy and Natsu kiss' and you'll find a variety of stories written by different authors with different takes on their relationship.