Understandably, everyone suggests his or her hometown, but I'm not sure if this does improve Minimetro regarding gameplay (and marketing, too).
If it's just about "hometown-ness", better add a map editor and an online map exchange. But stuffing the game with two dozens of maps just by vote and not by feature is consuming time that could be invested elsewhere to more effect.
Munich, for example, has been suggested quite a few times, but makes up a rather boring map with just the river running straight from south to north. Seriously, play Cairo instead: same layout, same game.
Berlin and Hamburg are more interesting choices. Berlin with the winding Spree river and one or two canals in the center, and the lakes east and west of the city.
Hamburg has the even bigger Elbe river, which divides into two streams just south of the city and joins again in the central harbour area, the harbour making up sort of a quarter of the city itself. Two smaller rivers join the Elbe from north and east, there's a big lake in the center and endless canals.
John_McFo posted a nice map (a few pages up) which shows just what I'd look for: Interesting topography.
There is no use in a "my city has an advanced (or unusable) metro system" quite a large number of all suggestions start with – the goal of the game is to build the metro system yourself based on almost random stations! Who cares about an existing system?
A good suggestion should IMHO include a topographic map as John's, preferably with the existing metro stations marked to get an idea of the needed density (or map size) in-game.
Now, apart from this matter regarding gameplay, I'd try to spread the maps a bit more evenly around the world. Surely there a more buyers to please in Europe and North America, but yet I think the rest of the world is underrepresented, especially South America and Africa with just São Paulo and Cairo.
Europe, on the other hand, currently has three cities. Maybe each continent or area should have at least two maps before adding more to Europe, even if this means adding cities which have no real metro at all. Bright new future for them! ;]