Skylake is a given, which should give 10-20% CPU performance increase, 40% graphics performance increase, and 1-2 hour extra battery life.
I think USB-C port will be upgraded to generation 2, which will double the transfer rate to 10 Gbps. I hope it will get a second USB-C port, but I am not placing any bets.
Also likely are aforementioned 720p iSight camera and upgrading Bluetooth from 4.0 to 4.2.
I think Apple will save often rumored Touch ID, Magic Keyboard key mechanisms, and Thunderbolt 3 for MacBook Pro.