
While I agree with the "flow" analogy, and with the barrier the old freight line north of Lehigh presents in Kenzo, I think you're still being a little overoptimistic wrt Camden. I don't think Aramingo is as strong a barrier as you perceive it to be -- between it, the freight line, and the El, it is actually the weakest (lowest) barrier bordering Fishtown/New Kensington.
Yorktown and the Allen Hole are definitely shaping development around Temple. Interestingly, even as Templetown merges with normative Center City redevelopment in Francisville, significant redevelopment is happening on the north side of campus, in th triangle bounded by Temple itself, Broad, and the train tracks. With Templetown there is an important proximity effect -- development is not pushing westward as fast as it once did, and I'll bet that (especially with improvements around the main gateway at Berks) development will push east of the train tracks and infill in between the various APM projects in the area.
Mantua and West Powelton seem to have become development loci mainly as a sort of "Drexeltown" analogous to Templetown. Still, Market remains noticeably underdeveloped between 46th and the Science Center. North Broad has the same sort of wedge, with underdeveloped properties as far south as Race. My guess is that it's because most of these properties need larger projects to make the numbers work, and until the adjacent neighborhoods are completely built out, the numbers won't. It is noticeable, for example, that many of Templetown's highest-intensity properties are located along 15th Street south of Oxford.
And as far as South Philly is concerned -- there are only two neighborhoods there still lagging: Dickinson Narrows and Grays Ferry. "Deep South Philly" is pretty much all middle-class as it is, although the retail down there is characteristically suburban.
I'm not entirely sure how large the "biking city's" radius is from Center City. But the Northeast Corridor and the old freight line are massive barriers, and I think for development to jump over them will require significant public investment sooner or later. But there's still quite a large chunk of North Philly that can infill before that happens.