Do you realize the current 90D "metering sensor" is basically an RGB+IR 220,000 pixel CMOS sensor? And that the 1D X Mark II, 1D X, 5D Mark IV, 7D Mark II, 80D, 77D, Rebel T7i/800d, etc. have similar RGB+IR metering sensors?
I definitely do. But it's not a DPAF metering sensor. As to the resolution, the idea was mainly that upping it to the megapixel range would improve feature detection, bringing it closer to what a DPAF main image sensor is capable of.
As for ripping out the mirror, it's called "Live View."
Note that I said secondary mirror. That is, instead of taking the light that goes through the semitransparent part of the main mirror and redirecting it to a PDAF sensor, just let it hit the image sensor (the shutter would be open like in full Live View) and let DPAF do its job. The idea is that OVF would remain available. The effective aperture and thus the achievable phase difference would be constrained, and there would be vignetting, of course.