Yes, there are quite a few. You can start by looking on popular fanfiction websites. Some might focus on their relationship dynamics in the context of the original story's world, exploring how their different personalities interact.
Well, it could be a number of things. Maybe she faces some challenges, has a major life change, or discovers a hidden truth. But without reading the specific novel, it's hard to say for sure.
Another option is Tumblr. There are often fanfic writers who post their works there, and you might be able to find some Himeno x Denji fanfiction by searching relevant tags or following the right blogs.
I haven't read this specific fanfiction, but usually, in fanfictions about Himeno and Hayate kissing in Pretear, it might explore their relationship deeper. It could be a sweet moment of their romantic connection, perhaps a turning point in their story where their feelings are more openly expressed.
Often, the plot might include some external factors that either push them together or create obstacles. For instance, there could be a villain who tries to separate them, but their love overcomes it and the kiss is a symbol of that victory. Or perhaps there are some family or social issues that they have to deal with, and the kiss is a moment of escape from all those problems in the fanfiction.
I have no definite knowledge about it. It could involve an exciting story, interesting characters, and perhaps some unexpected twists and turns. But that's just a guess.