I am now fairly confident based on evidence I don't wish to make public at this point that Apple is planning new (likely UIKit) Music, Podcasts, perhaps even Books, apps for macOS, to join the new TV app. I expect the four to be the next wave of Marzipan apps. Grain of salt, etc— Steve Troughton-Smith (@stroughtonsmith) ٥ أبريل ٢٠١٩
I have been able to confirm from sources familiar with the development of the next major version of MacOS - probably version 10.15 - that the system will include music, podcasts and independent television, but will also include a major redesign of the book application.
The new music, podcast and TV apps will be created using Marzipan, a new Apple technology designed to make it easier to move iPad apps to Mac without too much code changes.