An adaptor that will turn your small sensor camera into FF
It's been attempted before, firstly by Nikon who tried to produce a small sensor camera that could use SLR lenses. The same general approach is used by Olympus to produce their UHQ (ultra high quality) micro 43rds lenses based on SLR lenses with built in condensers, tuned to each individual lens. But they were only able to attain a 1 stop improvement in speed not the 2 stops that's theoretically possible (I presume they had to limit max aperture for the sake of quality).
But if the might and expertise of Nikon couldn't get it working any Olympus only got close then I would be surprised if the Metabones system didn't come with some drawback somewhere...
I'm also skeptical, but the fact that Olympus managed to get one stop with excellent quality (resulting in some of the best lenses around for any system) means that this may be doable for a full-35mm-frame to APS-C converter, as long as the full-frame system has long flange distance (that is SLR), and the APS-C system has short flange distance (that is, mirror-less). I don't know anything about optical design, but it could be the case that this turns out to be doable for SLR full frame to mirror-less APS-C conversion, and for DSLR APS-C to micro-four-thirds conversion, but not for any other pairings.