I think we're looking at Skylake, TB3 in USB 3.1/C, and a new chassis with thinner bezels, larger trackpad, and updated keyboard. Maybe a jump in the resolution (something like 3360 x 2100, to give 1680 x 1050 baseline workspace @ 2X) would be nice.
I'd be really surprised if we get discrete GPUs this go around. Skylake's new Iris Pro 580 is apparently fairly beefy (compared to past solutions). The 370X of this generation has had so many throttling issues documented, the highest-end Skylake Iris Pro chip may end up being comparable, maybe even an upgrade under very heavy use. I could see Apple spinning the "same or better GPU performance in a smaller package" type marketing message. Hard to say exactly how the 580 performs, since there's limited information out there.
Don't get me wrong -- as someone with a MBP with the 370X, I'd prefer to keep the discrete option, I'd just not be surprised to see it fall off either. The 370X has been sufficient for me, but I would like to see faster, cooler-running CPUs for VMs and development, so I'm probably upgrading regardless.
I'm a firm believer that Apple needs to revise their notebooks sooner than later. They're 1-2 chipset generations behind on all of them compared to their competition. Granted some of that falls to Intel's inability to launch the Broadwell Quad Cores on time, but the point remains.