You might be interested in what I'm setting to out build. I'm working on Archapolis, a city builder with real time traffic simulation and interior views of peoples homes (which you can customize/build yourself if you want). Very early stages of development still.

As for steroids, here's a tech demo of what I've been working on: https://www.youtube.com/watch?v=7q0l87hwmkI

I created a path finding algorithm that can simultaneously path 300,000 units to random destinations at a comfortable frame rate. Units can choose from any of the shortest paths between two points (there are many in a grid), and from those paths, can also choose the path that matches any preferences they have.

I'm sure other people will suggest this, but does Cities: Skylines with mods kind of solve what you're looking for (admittedly missing the "runs in the cloud" aspect)? Some of the builds people make with mods in that game are incredible.

For a single-player game like this, instead of having it always running, I wonder if you could simply design the game in a way that the next state is deterministically calculated based on the time. So the next time you start the game, it loads the previous state + current datetime in order to produce the next state.

This would be prone to manipulation, if someone were to change their PC's datetime, but would give the illusion that it's always running without actually doing so.

For me, it wouldn't run in the cloud constantly. I don't want to have unattended disasters to take care!

A massive SimCity, with at least SimCity 4 level of complexity, without city tiles, like with a whole world simulation? Sign me up!

That was the vision for SC5, if I rememeber correctly. Everyone was pissed off that the game required the servers and the servers would melt down regularly making it impossible to play.

Perhaps just ahead of it's time?

Some sort of a Tamagochi Sim City? If you don't login often enough your city dies?