Matt,
Your work and the work of others is really amazing. I love seeing the screenshots of progress. It's great to see the interest in the PRR.
As you asked for feedback, I would love to see Philly to Pittsburgh finished first. Once that is finished, or maybe even mostly finished and leaving some detailing to others as a WIP, then you could think about branching out with the other parts of the division that connected to the main route. If you stay focussed on the main connector, you won't get overly distracted by starting work on additional tentacles that spin off from the "Monster." For the moment, you could include short trackage that headed off to these other divisions, and have a portal (if portals ever get fixed) representing them.
You could spend a lifetime just working the Philly area, including the main port yards and the other routes heading off towards New York City, Bethelehem, Reading, and even southern Jersey. Some of the North East Corridor Route might help branch this area as well. It's easy to get overwhelmed and burn out. My suggestion would be to forge on with focus being Philly to Pittsburgh. Just my two cents.
The other routes that branch off could be set up as mergeable, or just left as separate routes to save space or make it easier to download. As you noted, I think it is a good idea for anyone thinking about working on the additional parts of the division to use similar scenicking items and trackage, in the possible event that they could be merged into the main route.
Regards,
Gary