Is it possible to create native WatchKit applications, which would be able to run without an extension running on a paired iPhone ?
Not at the moment:
Apple Watch requires the presence of an iPhone to run third-party apps. To create a third-party app, you need two separate bundles: a WatchKit app (that runs on Apple Watch) and a WatchKit extension (that runs on the user’s iPhone). The WatchKit app contains only the storyboards and resource files associated with your app’s user interface. The WatchKit extension contains the code for managing the WatchKit app’s user interface and for responding to user interactions.
But it will come soon :
The iOS 8.2 SDK beta including WatchKit is available immediately for iOS Developer Program members at developer.apple.com/watchkit. The WatchKit site includes programming guides, human interface guidelines, templates and more. Starting later next year (meaning 2015), developers will be able to create fully native apps for Apple Watch.