With the beautiful job 3rd Rail did with the SD7/SD9, a logical follow-on using most of the superstructure tooling would be the SD18/SD24/ATSF SD26/UP SD24b.
Another one would be the EMC BB-1800 Box cabs -- ATSF Class1, EMC Demonstrator, B&O, and CB&Q. The problem is you have four distinct superstructures with this locomotive class.