-Tokyo as mentioned a few times before, would be a cool small metro map, that, if you wanted to included some water could have a line go to mainland China.
-For a South American map, Rio de Janeiro could be a dark horse, especially if you add in a line or two through the Guanabara bay mouth, the Lago Rodrigo de Fritasand and other lakes around the area.
-As I was randomly screwing around the internet, I found the Santo Dominigo metro map which looks like it could be a good fit, it has several lines within a medium (Relatively speaking) area with several rivers.
-Thought it only currently has a bus system and does definitely not add location diversity, the metro area closest to where I am, Hartford CT has many rivers and a large metro area.
-Setting already suggested like Vienna and Munich would be interesting to see.
You are doing a magnificent job on the game Peter and I can't wait to see Mini Metro in the next stages of development!
(Edit: Having trouble with the new user only two links per reply limit, will only include Tokyo and Rio maps for now)