With the release of watchOS 2 is it possible to build an app that runs only on the Apple Watch?
That is—the app is not present on the iPhone.
Similar in concept to the way Workout exists.
Build app for Apple Watch only
The ability to build native standalone Watch Apps is reported to be for late summer.
Xcode 7b, as an example, only offers this for new apps/projects (also frameworks/libs):
"This template provides a starting point for an iOS application with an associated WatchKit Application."
Your watch app will still have to be bundled with an iOS app even with watchOS 2. It just won't require the iPhone app to do all the work like in watchOS 1.