Running out of ideas? Try looking at pictures of cityscapes for inspiration? Figure out what buildings are built where and why, and the different functions of elements you see in the photographs, such as big vents designed to keep air running on skyscrapers, power lines, antennae, lifts maybe?
Avoid burnt-out places if you want to focus on the buildings, less so if you want to give a general feel or impression.
I'd suggest you also check out environments on cghub.com and goodbrush.com. The first is a portal with a ton of amazing artists, the other is the homepage of Craig Mullins, an amazing painter/concept artist/illustrator with a lot of experience. There's a million ways to paint a city...