There is room to improve this game.
The city-building game involves creating different kinds of buildings on the map to balance the city resources. The creation often takes some time to complete. Players may need to wait for seconds or even hours. Some building games offer the option to eliminate waiting time during construction by letting the player pay a small amount of money. This is one approach to sell the premium feature on a free game.
We can add upgrades to our buildings. For example, we can introduce a solar power supply, which charges more but provides more power. The existing power supply may be upgradable to higher levels. We may even have some premium buildings that are only available to paid players. The premium model may use an in-app purchase mechanism while the game is distributed to app stores.