You can provide some easy variety by building some simple window frame shapes out of bricks and putting them on the sides of buildings. Tiny little ramped structures would help show entrances and exits to buildings, and window variety is easy. Just make a version of the print with lights on, with lights off, with blinds down, etc, and place them beside windows of different variants at random.
As well as that the window size on some buildings is really really weird, and some buildings have bigger windows than others. It just ends up looking very strange, you should try to rectify that.