MyMiniCity was introduced in 2007.
It is a web-browser based game where players can create their own virtual city.
Each city is given a web address where its progress can be accessed and viewed.
Anyone who visits your city’s site contributes to the development of the city.
One IP address can contribute to the development of a city only once a day.
No registration is required to use MyMiniCity.
In the game there are several development areas: population, industries, transport network, security, environment and business growth (trade).
Basically the web-owner has automatised web traffic generation to MyMiniCity website and has placed advertisements on it. Up until today, December 2019, it is getting around 50k+ visits per month.
This idea can be replicated to a more modern version with added twists and could make someone a big bank.
If you want to develop it, let me know, I can do advertising and marketing for it.