I'm am definitely in favour of having more commonality between classic and NG to help applications reach a larger audience. Something akin to the carbon library (in reverse) could make it easier to have binaries that run on both systems but with different outcomes.
It would most likely be more than a single library, and the minimum hardware spec on classic might be 040/25, but the benefit would also be a smoother transition from classic to NG as the apps you purchased will just run on NG. It would also create a brief market for 040 upgrades for classic Amigas.