I found out about Mini Metro just today; after playing for a while and seeing the tweet on city specific gameplay I must suggest Bogotá D.C. (which has just been mentioned before).
It is commonly said that Bogotá is the only city in the world of its size without an underground, and the current massive transport system is heavily criticized, working with a capacity that doesn't keep up with the needs of the city. You can see some (nice) pictures of it here.
Only buses run on the system lines, with one or two 'springs' or 'accordions' as we call them, for either single or double articulated buses ONLY.
More than a city, this is a suggestion for a different, fast paced, game mode, based on both limited carriages (and resources, inb4 local corruption) and heavy user base. You can see some rush hour pictures of the system here.
Now, it had been pointed out that Bogotá does not have any rivers across it, which clearly goes against the current gameplay, but I believe a mode with more limited upgrade options and a heavier contrast between rush and off hours would be interesting, just as Bogotá which besides its chaotic public transportation has plenty more know and love.