I guess people might argue this all day but I think Cities: Skylines is mostly that, with the rather large caveat that you can't start separate games on the sam

I guess people might argue this all day but I think Cities: Skylines is mostly that, with the rather large caveat that you can't start separate games on the same large map at the same time. You can still get mods (easily, from the steam workshop) which open up the entire terrain, and then use one of several different approaches to build multiple towns on the same map. I'm sure I'm forgetting other stuff that's missing, but they both scratch the same itch for me.