What about taking the most popular of the streamline cars (by actual use) and make them in 18" size? This would allow 10-6 sleepers, 36 or 48 seat diners or other sleeper variations. Standard seat coaches would be nice too.
Could offer each car in 5-10 roadnames spanning east to west. I would really like super detailed dining cars for a train since they would have the most interior detail visible through the windows (image each table with full place settings).
If anything, modelers can purchase a few cars to match with their current roster to make a more prototypical train.