If you need some ideas, go rent/borrow/pull out Ridley Scott's film, Blade Runner and spend some time looking at the city. Scott's classic vision is rather noir, but had become enough of an icon that people will think of it. With respect to your efforts, the building textures are a bit too uniform. And the choice of skies depends on how much atmosphere you want. Is it a hazy, damp place? Or is it drier? The more saturated colors will be more interesting to people, even if they aren't realistic. You know what's needed? A plume of steam (or two) coming from one of the buildings, to catch the light and make it more mysterious. i'd also place some fill and/or spot lights down amongst the buildings to give them some more depth of modeling. And then take that opportunity to play with the specularity of the textures: there's hardly anything shiny on the buildings themselves. In an animated clip, people might not notice these tiny things, but God's in the details. The overall effect will be improved.