Before you shoot me down, here me out. If Pentax does decide to go all in with a mirrorless system, they should consider using the 645 mount with a shortened flange distance. This would allow them the ability to only develop one line of lenses across all sensor formats, and also allow an easy upgrade path between sensor sizes. You could start with a APS-C camera, and eventually work your way up to a FF 645 sensor, all the while utilising only one lens mount.
If they were clever about this, they could still include an internal motor inside of the camera to allow use of they're extensive collection of screw drive lenses (with the appropriate adaptor). And for anyone worrying about size, here's a close to scale 645 mount super imposed on a close to scale K-70, to show that the size wouldn't be excessive.
The point that I am trying to make here, is that Ricoh could use the "mirrorless revolution" as an excuse to tidy up their mount selection, while simultaneously being competitive with modern trends, future proofing themselves by over enginneering their mount, whilst also supporting their current customers in all sensor sizes.