I never knew working steam and, all things being equal and if there was stuff available, would love to do a pre GSR layout maybe something offbeat like a WOI or Civil War theme.
The death of steam/new diesel era has a lot of variety as well, down at heel steam side by side with (a few) clean diesels.
The present day has relatively little interest for me, even station surrounds have become really ugly looking with Euro-cobble brick platforms, hideous palisade fencing and awful modern steel footbridges more suited to an oil refinery or chemical plant.