Actually, when the current era has no appeal to you, settling on a specific era is pretty simple, in my opinion. For most of us, I would think a period of time during our own life times, would be the attracting feature. It's the steadfastness to a specific date that adds the difficulty. In the April 2017 issue of Model Railroader, Tony Koester discusses this very topic in reference to the two layouts he has built. He says that Allen McClelland's Virginian & Ohio railroad influenced him, greatly. He liked how Allen attempted to emulate a real coal hauling railroad. The two became good friends and interchanged cars, to add to the realism of both of there model railroads. Although, sometimes it appears to me that Mr. Koester is so involved in thinking about his way of model railroading that he lacks the ability to see that others are not quite so strident and like a little more fantasy than he does. I still enjoy what he has to say!
What we need to be sure of is that we don't become so myopic as to loose track of the fact that this hobby means many different things to many different people!
Opinions given are my own and not meant to ruffle any feathers.
Northern Pacific, really terrific!