"Of course you know you opened a Pandora's box"
When I have an afternoon to devote to it, I will make a list, but to kick off (in addition to those interiors for PRB chair cars Otto mentioned, which I imagine could be modified to cars built from M&R heavyweight sides).....
A drop in coach interior for those wanting to "rebuild" MT parlor cars into coaches might not be amiss. Or for that matter, replacements for the coach itself, as over the years, many coaches were redone with lower density interiors- 66, 60, 54, 48 for instance.
I like the idea of something "modular"- even though it might not be as perfect as a specific interior for each and every design of car, it would let the design cost be amortized over many more units- which would help keep costs in check for all of us.