Honestly this mod really is a game changer I've been building train lines on maps like no man's land and the western wilds for the past few days. Several thoughts have come into my mind for additions. I know you're planning to add a Berkshire at some point, but I'm wondering about geared locomotives for logging. A two truck or three truck shay could be useful on logging maps like silverrun forest. Also if the issue you're facing with steam locomotives is sounds seeing how we haven't had steam locomotive sounds in farm sim before honestly i think i cam help because i have access to tons of sounds. Whistles bells chuffing sounds i can get it all 
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Howdy, glad you have been enjoying them!
Yes i am currently working on a 2-8-4 NKP Berkshire, it has been difficult to get the animations of all of the linkage to move correctly. Sounds have also been a MAJOR issue for me, trying to get realistic sounds and to adjust them as necessary for speed changes. I also have the UP #844 and UP #4014 Big Boy to add at some point as well.
I think a Shay loco would be very cool! unfortunately the linkage animations and gear animations and such are arguably more difficult than a standard steam loco, so if I am somehow able to find a 3d model of a shay, I think it would be a long ways off. but not impossible!
I can definitely help you out with sounds. I have a ton of steam locomotive sounds if you want me to share them, as for animation work id study animations and videos of the locomotives you're trying to model and possibly start off with something smaller like a switcher. Also for all your engines it might not hurt to find a way to state the minimum curve requirement in the store page, different locomotives need wider or tighter curves. And it would help people with designing their track layouts. Another engine i can think of that id absolutely love to see is N&W #611. #611 is my favorite steam locomotive and id be over the moon to have it in Farm sim. If you're looking for models I'd recommend turning to content made for Trainz. I've played games in that series for a long time and I've come to find tons of nice models.