The world is divided into a series of huge cities wherein time doesn’t pass. Outside the borders of these frozen cities, time flows normally.
The frozen cities are shining monuments to moments in time. Clean and tidy and the same day on repeat over and over again. The people inside never getting any older as they go about their lives living the same Tuesday afternoon over and over again. The world outside the cities is wild and rustic as the few who chose to live outside the cities age and grow their settlements over the years. There are specialty hotels partway in-between the cities that the occupants use when they want to age a few years or have children.
Unrest and upheaval are causing fights to break out between the cities and the border-folk. As those inside the cities work towards extending the time freeze past the borders, those outside begin plotting ways to destroy the mechanisms that keep time frozen in retaliation.
Your world is a powder keg about to blow; what do you do about it?