Ya... A multi-tiled city seems too local for a world-conquest game like civ. Besides, if a city grew to three tiles, lets say, and I captured one of the tiles, do you still get production? (or as much production?) and since I technically have a part of a city, do I get production? And how would borders work?
I think we should just leave it as one tile per city, and as it grows bigger, there can be skyscapers in it showing a higher population.