I'll selfishly throw out these two possibilities:
Baldwin DR-6-4-2000 / BP-20 Passenger Sharks. The Weaver versions are old and over-priced, usually beat up, lousy runners from what I hear, and not nearly as detailed as a 3rd Rail offering would be. I have to believe there are enough PRR fans out there to hit the minimum numbers based on recent projects like the Virginian EL-2Bs and the EA / E1 pairs.
Virginian EL-3A jackshaft electrics. If VGN 2-10-10-2s and EL-2Bs can make the minimum, why not these? It would be pricey as a set of three units but I believe no one has ever made them in O (MTH was going to years ago and then cancelled it). I get it -- they don't have the aesthetics of the streamliners because they're boxcabs. But you'll never get what you don't ask for, either.