Therefor for serious use I postulate some things which have to be happened until a Replacement of the mirror makes seriously sense:
- adequate electronic wysiwig-viewfinder
- permanent autofokus in the quality of the actual systems, incl. sensitivity under low ligh
- compatibility-bridge to the existing system
- new body-desing to bring serious advantage to the user.
The fourth point is a good one - I would prefer a box like design e.g. a cube of 8 x 8 x 8 cm³ with a high capacity battery and direct controls dials for aperture (ring around bajonet), Time, ISO, EV compensation. Such a camera with the shorty forty is very compact and fits into a lens compartment. The omitted prism and mirrorbox gives space for the rest - like the above mentioned high capacity battery.
... like the classical Hasselblad or Rollei medium format cameras ... and I remember the Rollei 2000 which was a 35mm camera with exchangeable magazins.