I'm not necessarily sure how I want the backgrounds to look, but here's some stylistic inspirations.
First is Chris Reccardi's and Lynne Naylor's short, "The Modifyers." I instantly fell in love with the backgrounds. The lines are gorgeous. And the use of shapes, especially in the buildings, is great. The buildings have great designs. The color palettes are gorgeous, but I feel they're a bit too mute for what I wanted to go for. Here's the short.
I also really enjoy Brandon Ragnar Johnson's backgrounds. His backgrounds feel like they're alive with great rhythm and movement. The colors are a treat, more along the lines I'm looking for. And he too works great with different shapes. My only gripe is that they feel too busy with details. Here's his blog.
I'd love to combine the first example's building designs and lines with Ragnar's sense of movement and color palettes.