It would be a bit pointless, since the theme doesn't really affect the game much. So a "Rome" version will have mostly same stuff just different pictures. Not such a great idea making same game with different design.
What can be nice is making a different city expansion that is also a stand alone game and combines with this one. Kind of like DOW the long night.
That's a good point! So...do you expect different cities to come out as an expansion, that is also a stand alone game?