Very excited about the upcoming iOS 14, macOS "Big Sur," and Apple Silicon. Really exciting times ahead!
iOS apps on macOS, but why not watchOS apps on iOS instead of widgets?
You don't need a watchOS app on iOS, they are specifically made for the watch tiny screens, and very specific features. If I were a developer, and wanted my watchOS on iOS, I would simply create one for it. I am sure it would be something rather easy to do. iOS apps (though I am thinking more as iPadOS apps) on macOS make sense. They are normally designed for an almost desktop-like interaction.
watchOS apps on iOS is a cool idea indeed, would be like having widgets on steroids. Maybe there are some limitations not allowing that specifically? Or they just didn't think about it...