Power, as in Watts is going to be an issue. Silicon semiconductors are hitting a power wall. I expect a future console would be limited to similar wattage as current gen (~150W max, ~80W idling). Could be even lower due to energy policies in some countries. That will actually be the constraint on how powerful a console in a box can be.
So within this time frame we might have dark silicon (transistors on a chip that can't all be turned on). That means you can build in redundant technologies that you turn on when you need them. If you need more CPU power you turn on some CPU cores, if you need more GPU then you turn off a few CPU cores and power up more GPU cores.
Performance scaling I think may come from more specialized cores. Think the way a GPU is order of mangitude more efficient than software rendering on a general purpose CPU. If we will have ray tracing, then I expect ray tracing cores (RPU?).
You can see the difference in efficiency of CPU vs GPU vs ASIC in digital coin mining, ASIC can be 10X or more power efficient than GPU, and GPU can be 10X or more power efficient than CPU for the same work (* with exception of coins designed to beat this pattern *).
So I expect we will see less silicon devoted to CPU, more to GPU and to special purpose cores & decoders on chip.
