Another interesting concept is to create an alternate universe where the power dynamics are completely different. Maybe the shadow organization is the ruling power, and the story focuses on how the world adapts to that. It could also involve the main character having to deal with different challenges in this new setup.
First, re - watch or re - read the original work to really get a feel for the characters, the setting, and the overall tone. Then, come up with a unique concept. It could be something as simple as a day in the life of a minor character or as complex as a new overarching plotline.
The Eminence in Shadow light novel is finished. It provided a conclusive ending for the characters and their adventures, leaving fans with a complete narrative.
The Eminence in Shadow manga is indeed still ongoing. The creators are continuing to build the world and the characters' adventures. It remains popular among manga enthusiasts.