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!