As a person who enjoys the look and feel of both the old and modern generations of weapons, but hasn't bought a new cod since MWR and WW2 and largely prefers the golden era, I think it's a shame that the old gen weapons (cod 4 – ghosts) are slowly being phased out and forgotten for their modern contempories. Imagine if we could choose which iteration of cod gun to use instead of only getting to use mw19 and onwards assets.
With gunsmith, I think it makes the most sense to have the premire weapons use assets from mw19 and after. My concept on how this would work is, for applicable weapons, a free blueprint would be added for everyone, that preserves the look, animations and sounds of the game it's from. (Examples in photos) As for inspects, which didn't exist yet, I think it would be fine to either disable them or just have them copy current gen animations, like how the non mw19 guns work now.
These older weapons are iconic and have a more smooth feel, rather than the hyper realism of current day cods and have legacies and memories that shouldn't be forgotten. With codm being a mashup of assets from over the years, I think this game would be perfect for bringing them back and bringing more personality to codm instead of the game being relegated to mw19 and onwards.