Any *body* R&D. In fact if Phase One had a liveview mirrorless body in the pipeline they wouldn't invest R&D time in this kludge. This looks like a solution meant to give liveview P250C users good wideangle solutions for a stopgap year or two until a real Phase body arrives that can take short-throw symmetrical lenses.
Frankly, I like the idea - superb lenses, superb backs, factory calibration, well-integrated software, iPad screen for liveview, evey part here is best of breed. Maybe Phase don't need a new body if AF or cadenced shooting is not necessary and shooters just use wides. I guess Arca now need to do a deal to bundle the $10 Hassy Cmos back 
Edmund
Honestly once you use a well tuned tech camera setup, for Landscape and Architecture, you don't really wanna mess about with a clunky MF SLR. I only use my Digital Back with an SLR when photographing people (mostly handheld) and works well for fast tabletop work also but thats about it for me. For wide angle work on a tripod of mostly distant subjects the tech camera is just much more efficient IMHO. You can rig up a pretty small and light package with lenses that are well above and beyond any MF wide angle lens made. The only MF SLR that isnt that clunky / cumbersome is the Leica S. But it is an integrated solution. The 645z still qualifies as clunky in my book but it is not too bad. It, like the Hasselbad H and the DF+, is still carrying the design burden of 645 film, a large Mirror Box and correspondingly long Flange Focal Distance.
But the same thing happened with 35mm SLRs. It was not until the Sony A7's and the Fuji X-T1 that we really got large sensor interchangeable lens cameras with very short flange focal distances and smaller and lighter bodies and lenses.
The Leica S is a step closer to that and as such it allows (with adapters) the use of a wide range of lenses. The ALPA and now the ARCA FPS are also a step closer to a MF "Mirrorless" Body.
Medium Format Digital will get there. But it is gonna take a while. I know it is a bit frustrating since the parts/technology are out there to make it happen right now.
