Newly discovered city (on a new continent or on a pre-existing one or even an underground city, it's up to you) with a functioning similar to the planet Krieg from the Warhammer 40k universe.
An extremely militarized city with extremely strict rationing and curfews.
The citizens would have no names, only numbers (like C-5487 or 2453-N) and they would have a total absence of family affiliation. They would have a very strong nationalism and pride.
All people would be coached and trained from childhood to become a useful citizen and jobs would be assigned according to the family's work (e.g. a worker's son would become a worker and a politician's son would become a politician).
People in government would also be trained from childhood to have unwavering loyalty to the nation's leader.
The city would be under a monarchical type of government with a royal leaf that would be long gone or extinct.
The city would also have a hunter's academy led by a director who would also be trained from father to son to become one. So very cold towards his students and very pragmatic do not hesitate to abandon these students in mission if it allows you to keep the city safe. The students would all have a military type uniform there is a strong resemblance with the uniform of the soldiers of Krieg and having all a gas mask and a shovel as essential. They would also be called by numbers but their letters would all start with "C" to show their affiliation as hunters.
The city would have been a colony during the Great War whose existence would have been lost after the war ended. It was originally built as a training center for soldiers for whatever kingdom you want. Isolated from the world it never took the end of the Great War and ants since the end of the Great War soldiers continuously.
(Scenario to follow or not)
Yellow arc, then on a mission with his team would have been overwhelmed by the Grimes. His team and him cornered he would have chosen to sacrifice himself as an appas to allow his team to flee The attracting further and further in the forest he would have been knocked out by an Ursa, but while his fatal fate approaching he will be saved by a team of the secret city and brought back in it. After many adventures, we discover that he is in fact a descendant of the leaders of this city who were Arcs and who received control of the city before the end of the Great War as a reward for their heroic deeds, and that he is therefore the absolute leader of the city by blood right. A few weeks after his ascension to the throne his former (J)NPR team searching for him then will accidentally enter a tunnel leading to said city.
I'll let you write a story with this idea, I found it interesting but not having the literary skills to write a credible story I prefer to let someone else try.
Translated with Translator (free version)