No one really knows for sure. There are rumors, of course, but they are to be taken with a grain of salt.
Just in terms of raw CPU or GPU performance, Kaby Lake will be a very, very minor update (unless you reeeaaaally care about 4K Netflix, maybe, if Apple implements it in Safari), and there's no word on a new line of AMD GPUs anytime soon. From that perspective it's likely to be very underwhelming. There are some rumors that the 15" might switch to desktop DDR4 to support more than 16GB of RAM (a dubious rumor to me, personally) and Apple may possibly increase the battery capacity on all models a bit by using a terraced battery structure like the 12" MacBook uses.
Altogether I wouldn't expect a huge upgrade. If you need a new laptop now, the 2016 models are fine.