Just read a paper from the same group that I did my PhD with. Most of the paper wasn't very relevant to my own work but I did find some inspiration.
The paper was Styers, D. M., A. H. Chappelka, L. J. Marzen and G. L. Somers, 2010. Scale matters: Indicators of ecological health along the urban-rural interface near Columbus, Georgia. Ecological Indicators 10, 224-233.
They looked at measures of road density and fragmentation along with population measures at different spacial extents. I was disappointed that there was any ecology in this paper. Oh well.
What Styers said that piqued my interest was this "Urbanization effects on natural resources extend well beyond the boundaries of urbanized areas into surrounding wildlland environments (Macie and Hermansen 2002)." This is cool and I haven't thought much about this. How far do urbanization effects extend beyond what is recognized as being urban? I imagine there are biological and abiotic effects that have the potential to participate in such a phenomenon. How far does the urban heat island radiate across the landscape? Or is it smaller than the urban footprint. I also would think that cats, dogs, and rats have an effect beyond the footprint.
Then there are questions of ecosystem services/processes. If urban areas have more invasive plants are birds moving them into the larger landscape? Are hummingbirds moving pollen out of urban areas? Lots of ideas.