Most abandoned cities are from people that sign up and then find they don't like the game so just never log in again, those cities should get removed after 30 days of inactivity. It's great when that happens but the problem is with the cities where the player has paid real money into the game for whatever reason or reached chapter 3 in the research tree, if a city meets one of those criteria it never gets removed. As
@LazyTony said there have been many complaints about dead cities but nothing ever gets done.
Perhaps, after a pre-determined period of inactivity, an email could be sent requesting the player to answer within a certain time. Needless to say, should it not be answered someone will say they did not receive the message, however, I believe there are ways to prove that it has been received. So, after sending the message and there has been no reply the city could be, 1) removed if no hard cash has been spent; 2) if cash has been spent, move the city to a "wastelands" where it can sit out of the way, a) forever, or b) deleted after a further predetermined time, when obviously, some records must be retained concerning the city and player
but not on the world map. I realize there may be legalities which must or should be observed at this time, but the organisers of the game should not stick their proverbial "head in the sand", a limit will eventually occur if nothing is done about inactive players and obviously stagnant cities. Should there be a reply then obviously the situation would depend on the contents of the message.