I think that a city should be able to be placed next to another city, but ONLY next to another city to create a type of "Super City".
It would cost more money, maybe [number of cities in Megalopolis] x 1, plus inflation, or something, but it would add on to the Super City's commerce/hammers.
Once the center of the city is taken, and kept for [# of parts of city], the city is theirs.
I realize that you could just go off and get a 20 tile super city and then destroy the competition, but every big city is probably going to attract a lot of terrorism, and would probably be the first target of an attack.
It would cost more money, maybe [number of cities in Megalopolis] x 1, plus inflation, or something, but it would add on to the Super City's commerce/hammers.
Once the center of the city is taken, and kept for [# of parts of city], the city is theirs.
I realize that you could just go off and get a 20 tile super city and then destroy the competition, but every big city is probably going to attract a lot of terrorism, and would probably be the first target of an attack.