The Atrix was exactly where I hoped phones were going and I was so disappointed to see Motorola drop it, and nobody else pick up (I couldn't buy the Atrix because it was Verizon only.) At one point Canonical had an alpha of a Ubuntu/Android hybrid which was intended to be similar, but that seems to have disappeared completely too.
It'd be piddlingly easy to do in hardware to the point I doubt it'd change the cost of the device by more than a few cents - make sure the USB port is bidirectional (it probably is already) and put in an HDMI out (maybe using MHL.) The software... well, as I said, Canonical already had something, Microsoft has Windows 10, there's a few prototype Android desktops out there which, if a community rallied around them, could be made usable.
This is not hard, it's just nobody seems to want to do it.