I'd argue against the underdog image as the central theme, but that's because I'm older than Sylvester Stallone. When I look at the "I Love Phily" poster Ironsij0287 posted, the elements that most scream Phily to me (and I've spent many weeks there) are the stars in the letters of the name. Phily is
most famous for being where the USA got its start. The Declaration of Independence was penned and issued there; the Constitution was negotiated and written there; it's where the first two Congresses met, including during most of the Revolutionary War.
I'm not suggesting that William Penn or Rocky be ignored, but that stars and bars be added. Whatever else you do, add more patriotic elements, such as bunting. Consider its value as the primary theme: design flexibility. You can make use of almost any size/shape space.
As a highly imperfect test of this theme idea I offer this graphic and ask:
when you see this, what city comes to mind? Now obviously, in the context of this thread, there's a bias. But maybe Boston or even NYC come to mind first. Both are larger than Philly and get more TV time on July 4th. But if Phily fits in there for you then my suggestion might hold some water.
PS: I'm surprised there's no scrolled parchment and quill in that Phily poster. That image is firmly in my gray matter when I think of Phily.