Here are some of my suggestions for K-5 II successor:
-Redesign the AF point selection lever - I don't really fell when the next mode clicks in. And the lever moves hard.
I've written about it in that thread:
https://www.pentaxforums.com/forums/pentax-k-5/209662-k-5-ergonomics-button-layout.html
-Another thing is fact that my K-5 is quite slow to response. Zooming during picture review is not as super fast as on other cameras. Camera doesn't let you change shutter setting for a brief moment after powering on. Applying in camera distortion/abberation correction is very slow. I guess that's due to the fact that the in-camera processor is not the fastest one. K-30 had a different processor and it's said to be much more responsive. God only knows why they didn't upgrade processor in K-5 II.
-RAW files weight a ton. In Photoshop you can resave them as compressed DNGs. They take around 70% of the originals. I think that there should be an option in camera to save compressed DNGs. Another good idea would be possibility to choose RAWs with bitdepth (12 or 14 bpp like some Nikons have). I guess you could save some space thanks to that.
To clarify. These are only little details. I guess most of the people don't really notice these things. But I look at the K-# line as an evolving family of products. K-5 II seems to me as an almost perfect camera. Just these three things that I would like to see changed in next iteration.