I think it will be a high clocked custom Raven Ridge cores. A nice enough single thread perf improvement over Kaveri Era Jaguar cores, while keeping thermal/power footprint low to balance out the increased power demands of the GPU.
8 cores of Ravens Ridge at a good enough clock is probably enough to cycle those draw calls in a parallel environment such as DX12/Xbox programming. These cpu are bandwidth bound, and it sounds like they'll have plenty of that around.
Not to mention I still expect some custom underpinnings by Microsoft to optimize a unified memory environment.
PC I i7 3770K @4.5Ghz I 16GB 2400Mhz I GTX 980Ti FTW
Consoles I PS4 Pro I Xbox One S 2TB I Wii U I Xbox 360 S