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.
A possible scenario in a fanfiction could be that Natsu realizes his true feelings for Lucy during a mission where he sees her in danger. He rushes to save her and in the heat of the moment, their lips meet. This kind of kiss is often a spontaneous expression of love and care in fanfiction stories.