Yes. The Yona of the Dawn manga has ended. Although it might be sad for fans, it left a memorable impression with its engaging plot and characters throughout its run.
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.
I'm not sure specifically what the 'shin jae ha girls love story' is about as it could be a very specific or personal story. It might be about the love experiences of girls related to someone named Shin Jae Ha.
It depends on your preferences. The anime brings the story to life visually, while the manga offers more detailed panels and inner thoughts of the characters.
One more theme could be the exploration of the fantasy world. Writers might create new areas in the 'yona of the dawn' world, or expand on the magic and mythology that already exists in the original story. This can make for some really interesting and imaginative fanfictions.