At the end of the WWDC Apple provided a sneak peak of a feature coming next year, iOS apps can be easily ported to macOS. This should increase the number of macOS apps available dramatically!
This was one of the persistent rumours leading up to WWDC but has been announced as a multiyear project with developers not having access until next year.
iOS and macOS shared a number of underlying frameworks but with this update Apple is bringing across more iOS frameworks to the Mac. Apple has used the new macOS Mojave to test some of the new iOS frameworks on Mac with Home, News, Voice Memos, and Apple Books iOS apps now running in macOS.
Apple is adamant that iOS and macOS will not merge but bringing iOS frameworks and apps to macOS should revitalise the Mac App Store which is far less popular than it's iOS sibling.