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.
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.
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.
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.
Themes in 'kiyo's story book story' could include nature. Maybe Kiyo loves the outdoors and has created stories around beautiful landscapes, animals in the wild. Also, friendship is a likely theme, as many stories are centered around the bonds between characters.
One interesting fanfiction could be about Kiyo and Megumi going on an adventure in a magical realm that's not shown in the original series. They might face new challenges together and discover hidden powers within themselves.
Since we don't have a lot of information about 'kiyo's story book story', we can only guess. It could be a story that Kiyo wrote to express their feelings, perhaps about friendship, love, or the challenges they've faced in life. It might start with an ordinary day in Kiyo's life and then take a turn into an extraordinary adventure filled with lessons learned and memories made.