How about some new engine sound sets for EMD diesels. This one-size-fits-all sound set used for like 10 years now on everything from the SD70ACe to the SD40 was never even close to as accurate as what MTH put in their PS3 1970s - present EMDs. The shame is that for all the money and effort Lionel has put into some great tooling for newer EMDs, they seriously hurt the appeal with poor or inaccurate sounds.
I know not everyone cares about this, but if you can't hear the clear difference between the 710 prime mover in a 70Ace and a 645 in an SD40, you need to get your ears checked! MTH knew the difference and the 2-rail O/HO/N/DCC guys are downright spoiled on sound accuracy.
Surely it wouldn't be too difficult to find a short line or museum with an operating SD40 or GP40 that a sound set could be recorded off of for a generic 645. A 710 might take a little more work, being newer, but there are plenty of 60/70 model EMDs around.
I could point out that the SD45 and SD80mac (both excellent models from Lionel) have 20 cylinder variants of their 645 and 710, respectively, and sound a bit unique compared to the more common 16 cylinder models, but I don't want to ask for too much...
An even cheaper option would be to allow the end user to change sounds like DCS and DCC allows.
And on the topic of sounds, why in the world when Lionel had a fantastic and accurate ES44-style K5HL horn in their die cast and heritage ES44 runs did they not put this horn on any of their later runs??