I'm picturing a game where you take the role of an arcology planner and are given a certain number of ingame years to prove the viability of the concept as fully as possible. Game takes place on an isometric grid, where you build your arcology out of predefined modules such as living quarters, industries, power plants, greenhouses, etc. You have to manage things like the flow of resources between districts, transit, waste management. You can connect your arcology to the wider world to do things like import resources and export waste, and you will need to do so especially in the early game, but you also gain a growing ability to handle these things within your arcology as you expand its capabilities. Maybe also have the player have to deal with structural soundness and making sure the arcology doesn't collapse. Scoring would probably be based on population, land footprint (smaller is better), and self-sufficiency.

Hmm. Maybe after Guardian Cry.