Although it makes some sense to align the brand names a bit more, I still see a problem with the version numbers. What OS X, iOS, watchOS and tvOS all have in common is that they maintain an internal versioning, but Apple is not using it in the same way when they refer to the products, i.e. sometimes they include the version number as part of the brand and sometimes they do something different.
Brand – version – referred to as
OS X – v10.11.X – OS X El Capitan
iOS – v9.X – iOS 9, iOS 9.1 ...
watchOS – v2.X – watchOS 2, watchOS 2.1 ...
tvOS – v9.X – so far just tvOS
If they indeed change the brand OS X into macOS/MacOS/Mac OS, then there is still the California-related name and that 10. If they decide to keep the California-related name, then the consistency is already gone and people will still call OS X by that California-related name. If they keep the 10, then there will be an awkward ‘macOS 10.12’ (or ‘macOS 10.13’ next year). If they drop the 10, then they will pretty much ’skip’ the 11: macOS 12. If they keep increment the major version 10, then they could also go for macOS 11. If they start over, then the versioning becomes inconsistent or confusing.
All of this just does not seem ideal to me. I wonder whether they will have something else in mind entirely, e.g. something that encompasses iOS too.