A tricky idea and a promising side line.
1. d2d4  1... d7d5 In a delicate match moment, Carlsen chooses to play a la  Nakamura. The American GM had won the first game as White and was doing  great in the second as well, probably missing a chance to increase his  lead. 2. c2c4  2... e7e6  3. b1c3  3... a7a6  4. c4d5  4... e6d5  5. c1f4  5... g8f6  6. e2e3  6... f8d6  7. f4d6 7.Bg5 is the modern continuation. 7... d8d6  8. f1d3 