1st, i like and encourage the idea of including the surrounding counties. philly is called the 5 counties, of philly, mont co, bucks co, chester co and deleware co, not to mention suburbs in de and nj. i was thinking you could break the counties into 2 or three, for example mont. co could have willow grove in the east, norristown in the center and k.o.p. in the west. maybe do something similar in the other counties, and maybe have each of those terr only attack a certain part of philly, or they have to go through their centers (county seat).
as to historical, yes philly has so many firsts it would be hard to decide how many you want to include, but you could do a map based on chronological geography. start with original philly, center city, and include the other early developments like germantown as increasing bonus, for players to gain they have to grow out, eventually into northeast philly and somerton, the last additions chronologically to my knowledge. maybe toy with doing a middle ages or 1st colonies type thing where you have to keep your early bonus. something like that would be possible, but the number of territories would have to expand.
but this map is great for a first try. i would lose the liberty bell though, that just seems a waste