Perhaps there were unused stories centered around a character's backstory that didn't make it to the screen. For example, a story about Odo's early days on Deep Space Nine before the start of the series, how he interacted with the Bajorans and other species when he first arrived. It could have shown his initial struggles and how he developed his views over time.
There are potentially unused stories that revolved around unique alien species. Perhaps a species that communicates solely through musical frequencies. However, the writers might not have been able to fully develop this idea within the framework of the existing Star Trek universe. Or there could have been a story about a Star Trek ship getting lost in a dimension where the laws of physics are completely different, but it was abandoned as it was too difficult to explain clearly.
Some of the highly regarded 'Star Trek Deep Space Nine' novels include 'A Stitch in Time' by Andrew J. Robinson. It gives great insights into Garak's backstory. Another one is 'The Soul Key' which has an engaging plot filled with the political and spiritual intrigue that the series is known for.
The novels also allow for more in - depth exploration of themes. In the 'Deep Space Nine' series, there were a lot of political and moral themes. The novels can take these and really dig deep, like looking at how different species interact in the context of the Dominion War. They add layers to the relationships between characters, races, and different parts of the Star Trek universe. This enriches the overall franchise by giving fans more to think about and more stories to enjoy.
One of the main themes is exploration. Just like in the Star Trek universe in general, characters in the Deep Space Nine novel often embark on journeys to new planets and encounter strange new species. Another theme is diplomacy. Given that Deep Space Nine is a space station in a strategic location, there are many diplomatic negotiations and power plays between different species.
One interesting 'Star Trek Deep Space Nine' fanfiction could be one that focuses on the backstory of a minor character like Nog. It might explore how he grew up on the station and his early experiences with the Federation crew. Another could be a time - travel fanfiction where characters from different eras of the show's timeline interact in unexpected ways. And there are often fanfictions that center around romantic relationships between characters that were only hinted at in the original series, such as a more in - depth look at the relationship between Kira and Odo.
One way is to look at interviews with the writers and producers. They sometimes mention ideas that didn't make it into the final product. For example, in an old interview, a writer might say they had a concept for a new alien race but it got scrapped.
The Emissary is surely one of the main characters. There are also likely to be representatives from various alien species like Klingons, Cardassians, and Bajorans.