You do know of the Sony RX1, of course. FF and mirrorless.
Why not interchangable lenses?? Its impractical to make ultra wide with the short flange back, and a complete new 20K set of high end lenses will take years to introduce. You could adapt conventional FF lenses, of course, and that might be the only practical way.
But then, whats the point? Extreme slow AF is not for everyone.