I have been digging, and came to conclusion, that if Raven Ridge APU will have HBM2, and it will be available to both: CPU and GPU, that means we are looking not at 4C/8T + 16 CU MCM package, one, separate design.
HBM2 memory controller must be on the die, to get it functional for both: CPU and GPU. Overall die size should not be bigger than 220mm2 for 4C/8T + 16 CU design.
Ryzen CPU is 180-190 mm2. So there is some of room to play with for "Frankensteining" the Raven Ridge APU from Ryzen + Polaris dies .