I have listed some elements in the middle that I am looking to incorporate into the layout.
Those elements are at the most basic level. I would suggest getting a bunch of layout books from the library and examine all the track plans. Start picking out larger elements that you see and like from them. After assembling a list of those elements then start working them together into your layout.
For example in the May 1970 issue of MR there was a track plan for the Cumberland & Susquehanna that had two towns in it that really captured my imagination. I have incorporated elements of those two towns into many of the layouts I have designed since then. In fact, that might be an excellent layout to look into for ideas for an Allegheny flavored layout.
1. With that much space in N-scale there are many possibilities, so don't jump in on the first good idea that comes along. Get many good ideas and consider before choosing and starting to build.
2. Since you want to have a "rail fan" type layout, I would suggest making two levels for lots of train viewing, but also inclusion of many operational features for use as your model railroading appetite matures and just watching trains orbit becomes boring.
3. The Allegheny Mountains! 1960! Why go freelance? My heavens I would suggest just backing up two or three years and model the N&W. They ran their last steam locomotive in March 1960. My second suggestion would be to "model" the V&O. Freelance work takes so much research, thinking, and planning just to get a 1/2 way reasonable back story. Researching a Prototype is much easier. Were I starting totally from scratch, right now, I would model the N&W Radford Division & the Abingdon Branch.
4. Leave lots of room to park trains out of site (commonly called a hidden yard or "staging"). That way instead of watching a single or two trains circulate they can stop and a different train be brought out without having to manually take one off and put the other on. I.E. Instead of watching coal train, coal train, coal train as it loops, Watch coal train, Pocahontas, mixed freight, express reefer, coal train...