I like this idea a lot but the two biggest problems that comes to mind are that the packs would get expensive quickly for people buying when you add up a bunch of different model types and also all the subtle differences within each locomotive type.
The latter is actually the primary reason we tend to shy away from shortline rosters with a bunch of one locomotive type that is compiled of different lineages. A roster might be a bunch of GP40's on paper but they will end up being some ex-penn central phase 1a, some ex-rock island phase 1b, some ex-southern pacific phase 2b, etc. The little differences add up to a lot more work if we were to model an entire shortline roster, and more model work leads to more areas for mistakes, more time spent fixing things for late-development discoveries, more cab shells taking up space, it's just not worth it unless someone commissions it and foots the bill for the extra work.