Ming-Chi Kuo is about as respected an Apple crystal ball gazer as there is, and he sounds pretty sure about a redesign for the 15". Yes, it seems a year early, but there are reasons for it. The keyboard isn't popular and the 6-core processors are testing the thermals of the present chassis.
I wouldn't be surprised if Intel caught Apple off guard by increasing the core count of the 14nm Skylake derivative mobile processors, and even more so by doing it twice only a year apart. All other things being equal, an 8-core processor is going to use more power and produce more heat than a 4-core processor using similar cores and based on a similar process. Intel hasn't increased TDP, but they have increased actual power use and heat dissipation by running the processor over TDP more of the time. The present 15" MBP was not designed to cope with these higher-power CPUs - Apple was betting on 6 and 8 core 45W parts being cooler 10nm chips.
Apple can't wring the 10nm chips out of Intel any sooner, they can't ignore the hot-running 8-cores, which are likely to provide very real performance improvements, and they may very well not be able to cool the 8-core chips in this chassis. They can, however, design a chassis that can handle them. They probably realized a couple of years ago that they were going to get a couple of generations of hot-running chips as the 10nm project stalled. Assuming that they started to design the 2016 chassis in 2014 or so, they probably saw a roadmap that looked something like this for 45 watt chips:
2016: quad-core 14nm
2017: quad-core 10nm
2018: 6-core 10nm
2019: faster 6-core 10 nm (next generation microarchitecture).
By 2017, a customer the size of Apple might have seen a road map something like this (even if Intel wasn't publicly admitting it):
2017: extra generation of quad-core 14nm
2018: 6-core 14nm (a chip Intel hadn't intended to produce)
2019: 8-core 14nm (the only way to go faster than the 6-core - there's simply no more headroom in Skylake)
2020: Finally, a 10nm 45 watt chip (and with a couple of other improvements rolled in).
Apple probably asked some hard questions like " how real is that 10nm chip in 2020"? That answer isn't public yet - I'm sure Intel will get a bunch of 10-15 watt chips out, but the 45 watt model? Will we see yet another 14nm generation in 2020?
They also almost certainly asked "just how much power can those 6 and 8 core chips really draw"? They didn't like the answer - it was probably something like "the 6-core can go 60 watts or more, with the 8-core pushing 70". When Apple heard that, they realized they had to push the chassis redesign up a year, because it is something they can control. They aren't going to get a 60-70 watt CPU into a chassis that was designed for a well-behaved 45-watt CPU.
That said, I expect a nice machine something like:
16"+ 4K (or higher?) display - remember that the original Retina burst upon a world of Full HD displays at the most, and even the workstation laptops had lower resolution options - Full HD (occasionally 1920x1200) was an upgrade...
9th Generation 8-core CPUs
Minimum of Vega 16 and 20 GPUs (maybe a next generation that is similar but 7nm - that would also help the cooling - the Vega chips are already easier to cool than the old 560X)
32 GB RAM standard on top model (maybe a 64 GB CTO upgrade possible)
Same SSD options, probably somewhat better standard configurations - 512 GB standard from the bottom of the 16" line, perhaps 1 TB standard on top model? Will 2 TB and 4 TB options be better priced?
Some form of Touch Bar or (unlikely) fully remappable keyboard.
Probably an improved butterfly keyboard