Is it possible to use CoreBluetooth in a WatchKit extension for the new Apple Watch Series 2? Or, exist there other possibilities to connect a third-party watch app to a Bluetooth device without using the iPhone as a middle instance?
That isn't possible for the first Apple Watch, but maybe it has changed with the new Apple Watch.
Edit: CoreBluetooth is supported by Watch OS 4.0 and higher.
Unfortunately there is no way (yet). CoreBluetooth is still absent in watchOS3.x – hence you would need a wifi device if you want to skip the iPhone.
Update: Please open a rdar (Apple bug report), if you want to see CoreBluetooth in a future version of watchOS.
2nd Update: Don't bother opening more bug reports... it's coming in watchOS 4