It might make sense to go with either Star City, Central City, or Keystone City. The former is the traditional home of Green Arrow, and is just up the California coast from Coast City; Central City and Keystone City straddle the Kansas and Missouri border, and are the traditional home of the Flash family.

Most of the adversaries for Green Arrow and the Flash are "crooks with gimmicks" as opposed to people with full-on superpowers, which would make them a good proving ground before coming to Gotham City. Because Metropolis and Coast City have much more powerful heroes, their villains are usually of the same calibre, so Gotham might seem like a step down from those places.

I'm planning to have Barry Allen transfer from the Central City Police Department to Gotham, since he's an expert in investigating crime scenes that have a villain/vigilante twist. I'd be more than happy to have some sort of Guy/Barry backstory... or you might want to go with Star City for a bit of variety and Green Lantern loyalty.