Oh boy where to start:
Features I would ask for the following:
Option to mount Kaydee couplers upfront on steam engines and between diesel units.
Blackened vents on diesel units
Air release sounds on diesels in motion
Bring back sound and smoke in dummy units the CC2 units had them. What happened?
Legacy talking caboose that echoes talk with the locomotive.
Yes on the fixed pilots
weathered trucks on diesel units
Shades on steam engine cabs
What happened to the S3 adjustable draw bar? Shouldn't that be standard?
Bring back the Station Sounds diner and create a talking station that talks back during departure and arrivals.
Simple sound cars for small back ground noises. refrigerator noise, click clack of the wheels, simple sheep, or cattle noise modules that are easy plug ins to freight cars.
How about an operating Mail car? My American flyer car had that 50 years ago...
How about steam engines with a random steam leak with "break down" talk like the B units had.
Well just some ideas