is it possible to develop cross platform apps with ionic (2) or similar cross platform frameworks for the pebble watch with the pebble sdk?
The SDK is available for native android / iOS, so a possible way is to use that SDK library. But how should I realize that?
Ionic/Cordova cannot run native SDK's as they're still ultimately written in Objective-C/Swift for iOS and Java for Android.
Ionic/Cordova does not run any of your application using native code (ie. your code does not become Obj-C, Swift or Java, it stays as Javascript). It runs your app within a WebView as a Website. So really its not possible unless they offer an SDK for Javascript or someone has written a Cordova plugin for it.
If you're open to using a different Mobile Cross Platform Framework, React-Native will support custom Native Modules. Essentially, you can write your own Native Code that will be available to your Javascript.