Heritage cars would be great. A base already exists for some. SP 10-6 sleeper from the Sunset Limited project, dome coach from Empire Builder, GN Great Dome, Milwaukee Super Dome, (one or two used on the Auto Train) and of course, the Slumbercoach, just to name a few. Tooling would have to be modified to represent the changes the cars had under Amtrak, like the trucks, the addition of marker lights, and appropriate HEP detail.
I think a UP Pacific-Series 10-6 sleeper (built by Budd) would be best to start with. Lots of paint schemes that could be offered-UP of course, Amtrak Phase I-IV, VI, and the paint schemes of privately owned cars. Re-tooled SP Sunset 10-6 sleepers could be done alongside or after if the UP ones did well. That prototype was in Phase III only to my knowledge, but Birch Grove made it to private ownership. I believe it’s now in use on CSX’s business train. Another good 10-6 prototype would be the ATSF Pine-Series. Many of those cars served faithfully in Phase III, eventually being converted to crew dorms, serving into the Phase VI era in some cases.
Re-tooled Hi-Levels would be best done combined with Superliner Is imo. There’s Phase III of course, but select Hi-Level coaches were painted in Phase IV for the Heartland Flyer, and the Parlour cars made it to Phase VI.