We already have OpenTTD and OpenRCT. Maybe someone somewhere is creating an C++ remake of our favourite isometric city simulator, with multiplayer support!
But to do it right, I'd want it to be the scale of a moderately large nation like the USA. You'd buy plots of land one square mile as you expand. The game world would be seeded with a major highway and railroad system that you could not remove, just add connections to.
I'd want traffic to be simulated in the same depth as Cities: Skylines, and have freight have to move over a thousand miles to reach destinations.
But most importantly, I'd want the entire game world to be seamless. You should be able to pan from one side of the country to the other.
But such an ambitious game would require immense processing power. I don't know how I would scale it.
I am asking, because I think I haven't seen the multiplayer support in any railroad/transport-tycoon game. I remember in Railroad Tycoon 2 and then in Sid Meier's Railroads [0] we could play against NPC. Same as us, NPC is establishing a company and we may buy all his stocks to win.
[0]: https://store.steampowered.com/app/7600/Sid_Meiers_Railroads...
In OpenRTC, each player gets full control over managing the same park from a shared pool of funds. Mayhem may ensure if you set your game to public and don't configure permissions carefully.
In my mind, such an OpenSC2K's multiplayer would work like the latter. Each player managing the same city. Though if they removed game engine restrictions, it would be nifty if each player could manage his own city on one big shared map, similar to OpenRTC.