Last year I upgraded from an 11" Air to a 12" Macbook (rMP). Apple fit a bigger display into a laptop of similar dimensions, and it weighs less too. I think people (including myself) expect to see similar improvements on the MacBook Pro line, whether larger screens in the same dimensions (perhaps 16:10 like the rMB) or smaller chassis' housing screens of the same size.
My vote would be for 16", DCI-P3 gamut, 4K+, anti-reflective improvements, and True Tone.
Others may see USB Type-C as a reason for a redesign -- the new universal port that is smaller than USB Type-A. But the Alpine Ridge chipset only supports two Thunderbolt 3 ports. It could be pretty confusing to have additional USB Type-C ports that aren't Thunderbolt enabled. Would Apple do that? Upgrading from Thunderbolt 2 to 3 is already going to require adapters for DisplayPort, much less adapters for everything else. A full compliment of ports makes sense for a professional laptop, including beefy ports like HDMI 2.0.
I think it's important to keep in mind that the new rMP design was enabled in part by the new Core M processors. They sip 5 watts instead of 15 watts like the Air and don't require a fan. The CPUs on the MacBook Pro still need the same amount of power and cooling as last year.
And as much as I like the tapered design of the Air and rMB, I would hate to see the MacBook Pro become thinner if it compromised the performance with throttling like the first 5K iMacs. Nor have I gotten use to the shallow tracking of the rMP keyboard. It works well enough, but it still feels like a compromise.
Besides the possibility of an improved screen, I'm most excited about the internals. If the high-end MBP includes the upcoming R9 M480 GPU (or R9 M480X as a CTO), that could be a significant improvement, much like the current generation iMac dGPU in a power envelope suitable for a laptop. Skylake brings faster DDR4 memory, hopefully more of it, and maybe we'll even see 2TB SSDs.
Those are the things worthy of the Pro name. If you think about it, the recent 9.7" iPad Pro has a better screen, better speakers, and better internals than the iPad Air 2. The dimensions are the same.
But I wouldn't be surprised if Apple outs a larger MacBook (non-Pro) as well.