would this idea clutter the map? yes.
would a lot of people complain? yes.
could it be useful? yes.
could it be utilized in an interesting method? yes.
should it be done? maybe.
roll with me on this... ok, so you have a city you just made, you don't like the placement of it... so you abandon it. now one of two things could happen...
A) it could be left there to take up space and do pretty much nothing until someone decides to come along and take it (highly unlikely)... OR
B) if the city has over a certain resource output and other miscellaneous building levels... it could become an aggressive city... MEANING... (lots of more
fun coding, but it would be interesting to see) it becomes barbaric, so the city would thrive on it's own... it would create it's own units... hunt its own units... capture its own oasis... attack other players of the same level... basically PLAY the game on it's own... and you can give these barbaric cities higher levels than just 20... so it makes them a challenge, meaning if you invade a barbaric city of say 25... you'd have a 60% chance of looting a nugget, 40% gem, 20% medal, 10% scepter, and like 2% crown. (note: all these numbers are chosen randomly off the top of my head for EXAMPLE PURPOSES ONLY.)
plus to avoid something like city distance walls (intended @Backburner) you could have a "set aflame" option... meaning it destroys the abandoned city clearing the spot of it. or better yet when abandoned instead of just a carcass of a city staying there it could change to some sort of oasis? for example: you have lvl 15 iron mines (maxed out without coins) when you abandon the city there is maybe a 40% chance of it becoming an iron oasis? or just turning into an empty field again.
abandoning cities is an interesting idea, lots of loose ends that need to be tied up though... maybe my ideas added to others could possibly help further discover a better answer for it? but first... let us worry about abandoned accounts cluttering the map before we worry about abandoning cities.