This is just some subtle sketch for a possible city-building game with AI.
First of all, the base of the base - the building itself. What if AI could generate buildings right during the game? Even if we're not talking about "real" creativity, but rather about the ability to construct an object from pre-fabricated 3D chunks? Imagine, that you can describe, how you want the building to look in prompt and AI makes that building!
Second, the whole block\street. Imagine, you're just building the roads and marking the place, where you want to see your buildings and describe it in a prompt, and later AI populates that plot of land! Entire areas of turnkey property!
Third, the fluid-ness. What do I mean? You see, in ordinary city-building games, you're either choosing the building from pre-made assets or, in the best case, marking the space near the road to be populated with pre-made assets. Imagine marking a huge plot of land and writing in the prompt: "Make me a MegaTower"! And here it is, the giant monstrosity! You can make any of your wishes as big or as small as you want! Big power station? No problem! Tiny high-rise office? Here you are! Elite housing made from the whole mountain? Sure!
Fourth, the game scenario. Imagine, you're just placing houses here and there, and suddenly, the radio talks about a UFO invasion! Police and army trying to shoot aliens and saves abducted, flying saucers taking the peoples out of their homes and destroying property, while all emergency services working around the clock! Absolute chaos! What if your tropical paradise with a bungalow gonna get hit by tsunamis? Or a more down-to-earth scenario, with mass riots!
Fifth, more realistic everything, from the weather and day-night cycle to people's behavior. Imagine watching road accidents! Black Friday madness!