A report by Digitimes claimed that Apple is set to launch a new model of MacBook soon, potentially before 2015. The same report also claims that Apple will retire the 13-inch non-retina MacBook Pro by the end of this year.
Though it is unclear on which Macbook will be thinner, the current MacBook Air tapers from 1.7cm to .3cm, while the MacBook Pro Retina has a thickness of 1.8cm. So it might be either one of this two.
There has also been a rumor that Apple will launch a 12-inch Retina MacBook Air with a buttonless trackpad design and without a fan.