Get Sony A7R II at B&H Photo.
Pixel shift is a technology available on the Olympus E-M5 Mark II and on the Pentax K3-II as of today. It moves the sensor in single pixel increments in multiple directions, building up a composite image (in raw format or JPEG). The multiple samples (exposures) increase total exposure time, but reduce noise.
Pixel shift avoids / mitigates the RGGB Bayer matrix limitations, thus providing true color at each pixel and lower noise, and with enough shifts, increased spatial resolution as well. It requires a non-moving subject and camera and can produce ugly artifacts if those conditions are not met.
Though pixel shift is not a panacea and is tripod-only (at present at least), it is an awesome feature that is oddly lacking in the Sony A7R II, which has its IBIS feature (In Body Image Stabilization).
Conspicuously missing from the Sony A7R II feature set is Olympus-style pixel shift, which in my testing can deliver roughly a 32-megapixel image from a 16-megapixel sensor. On the 42-megapixel Sony A7R II sensor, a similar technology could in theory deliver ~80 megapixels (or 42 without Bayer artifacts), although a lens of Zeiss Otus grade will be mandatory.
It’s a pity that Sony is not offering pixel shift. Perhaps it will appear in a firmware update, but that seems dubious, since it is a terrific feature to brag about at a product release (and would be a first for a full frame camera). It might also be a problem to implement with the current Sony 11+7 bit raw file format; it demands a true 14-bit format, if only not to waste the huge bump up in noise reduction in dark tones.
If Sony does not implement pixel shift, then the most likely candidate is the coming Pentax full-frame camera, which utilizes the Sony 42-megapixel sensor (pixel shift already exists in the Pentax K3-II). And then one wonders about Nikon, which at the least ought to offer a Nikon D820 with the new 42MP Sony sensor, though we can hope for more.