Here's another one that would sell: the Evans Auto Loader from the early 1950's. The manufacturer would also have to come up with some 1/48 autos to put on it, since 1/43 cars won't fit. I'm convinced an Auto Loader would sell - train guys like cars too, and it would be a great addition to any transition-era freight train.
I haven't read through all the posts yet, so someone may have mentioned this already, K Line these cars, and did a very nice job on them, They were available in Union Pacific, NYC and Pennsy as well as Coca Cola and maybe some that I am not aware of. The K Line cars come up on the secondary market quite frequently, and usually get $65.00 and above plus shipping.
Having already been produced, they don't fit the topic of this thread, but if someone was to acquire the tooling for them, I believe they would sell well, especially if more road names were offered.
Doug