MartyE,
I so agree with you. I have the Visionline Cattle car, and the TMCC crane and boom car. They add a lot of fun to a layout. I think these types of cars, that add additional sounds to the train and animation, just add to the play value.
Along your suggestions... How about a Legacy milk car that has a new mechanism which pushes the cans out smoothly instead of the solenoid.
The newer style operating box car which unloads the crates is really nice. Legacy control would have been even better.
For their fastrack line they came out with command switches. How about command UCS tracks for their older operating cars?
They should also add some more sounds cars with the wheel squeal. It makes the train sound much more realistic when you hear the metal wheels squeal as they go around a curve.
PRR use to fill their tenders by use of the drop scoop to reload water. I bet they would be a really neat feature if Lionel could implement that.
So many untapped ideas.