If I've played the piano for 6 six years and want to play a new instrument, is the violin a good choice?

I can guarantee you that you haven't mastered the piano if you've only been playing it for 6 years. It might make sense to start another instrument if you have enough time to continue playing piano. The violin could be a good choice depending on what type of music you want to play, most music doesn't have violin in it so you may not be able to play music that you listen to.

It depends on the level of talent you have, and how well that transfers to the violin. However the is considered to be far from easy to learn. A better choice might be the guitar, mandolin, or something from the woodwinds or brass. As far as having mastered the piano, I very seriously doubt that you have come anywhere near mastering it, and all of the techniques. Even the true master pianists never truly master it, with all of the various styles or techniques. These are players with twenty, or more years of studying, and playing, and are professionals. After six years, you haven't but barely scratched the surface.