Recently I've been doing research on Chicago Midway's aircraft operations in the late 50's and early 60's (right before O'Hare opened up)
Here are some retro models that I would like to see produced in 1:400 scale (I know a lot of these have already been produced but they seem to be quite rare):
Lockheed Constellation- TWA, Air France, Lufthansa, Braniff, Capital Airlines, Eastern
Vickers Viscount- Capital Airlines, United Airlines, American Airlines, Trans Canada Airlines
Boeing Stratocruiser- Northwest Orient
I'm sure there's more but those are all I can think of right now.