I recently got an Apple Watch. But I did not really like the now playing app hidden in the dock. So I made a new one for my Spotify iOS app with a complication. I am not a pro coder, but it was pretty easy to hack this together even for me...
The difficulty with making a stand alone now playing app is that it has to wake up it's iPhone counterpart to send and receive the now playing info. That could take a few seconds if it's has not been active recently. So it would be slower in those cases then just go to apple's own app in the Watch dock.