Yes, there are many. You can start by looking on popular fanfiction websites like Archive of Our Own. There are often stories that focus on the relationship between Hak and Yona and might include a kiss scene. Just search for 'hak and yona' in the search bar and then browse through the results to find ones that seem interesting.
Maybe in the fanfiction, Yona gets pregnant unexpectedly. At first, they are both shocked but then filled with joy. Hak starts building a small house for them to live in. They go through the normal pregnancy experiences like Yona having cravings and Hak trying his best to fulfill them. Their friends also rally around them, offering support and advice for the upcoming new life.
Definitely. Throughout the manga, it's clear that Yona's love for Hak grows and becomes an important aspect of the story. Their relationship develops in various ways that show her affection for him.
Another theme is growth. Yona's growth as a person and a leader is often intertwined with her relationship with Hak. As she becomes stronger, their love also evolves. In some fanfictions, it's shown how this growth brings them closer and makes their bond even deeper.
One common theme is adventure. Since the original story has a lot of adventure elements, fanfics often continue this. Yona and Hak go on quests together, facing enemies and exploring new lands, which strengthens their bond.
A common characteristic is the strong emphasis on their loyalty to each other. In these fanfics, Hak is always by Yona's side, protecting her no matter what. Yona also shows her trust in Hak.
Archive of Our Own is a great place to start. It has a large collection of fanfictions and you can easily search for 'hak and yona' related stories. There are tags and filters that can help you narrow down your search to find ones with a kiss scene.
Their friends would likely be very excited. For example, Yun would probably be all about making sure Yona has the proper nutrition during her pregnancy. He'd be researching all kinds of herbs and foods that are good for her. Jae - ha might be a bit more teasing towards Hak, but also very protective of Yona. And Zeno would probably just be happy and offer his simple but warm support.