Having a Swift implementation that talked to GNUstep would be a great thing, IMHO, but the developers do not seem to see it as especially important, and as I understand it, as an Apple project, it is a moving target and the Apple developers are not likely to accept contributions from external developers.
macOS is the “successor state” of NeXTSTEP if you will. Adopting some of the newer APIs and Frameworks could be useful.