Is it possible to go beyond Six Sigma? Well, yes. That's because its paradigm is linear cause and effect, whereas the world operates to a large extent on circular cause and effect: feedback.
Here's an overview of that issue and a few Six Sigma shortcomings.
Beyond Six Sigma: Systems Thinking and Six Sigma Shortcomings (pdf, 92K)
Beyond Six Sigma: Systems Thinking and Six Sigma Shortcomings (doc, 113K)
Don't take this the wrong way. Six Sigma has some great techniques; it just doesn't go everywhere it needs to go. In fact, it doesn't go to the most important places to overcome barriers to improvement.