Building on what you had in mind when you created
Philadelphia you could have a Main Forum that encouraged separate Threads for specific cities. Everyone has local spirit ... not just the Philadelphians.
It's not that these threads would be restricted to the local inhabitants, but the discussion might be relevant to attract new groups to connect within the larger community.
Toronto the Good comes to my mind.
the City thread starter could get creative with the title based on local lore.
City of Angels
This is similar to the social motivation of Derek Powazek's City Stories Project websites (
www.citystories.com).
I'm not suggesting a separate forum for each city, but separate city threads within a
Cities Forum.