Steve,
The beauty of being John Parker is you get to deside what kits you want to produce. Every time I see him I ask about kits that I would love to see done too. He just keeps telling me "I will get around to those - someday" And I think he will.
He told me last week that after the C-16 he plans to go back to Standard O and produce a USRA Double Sheathed Box car. He's been wanting to do that one for a while.
By the way I've been bugging him about the 6500 Flats and San Juan Passenger Cars.
The 4000 series would make sense too - after the first plain jane 4000 a whole series of work car kits could follow without much more die cutting. (Just like the work flats from the 6000 series kit he did)
Kevin