Well it probably wouldn't be too difficult for Pentax to accomplish. They already have a lot of those bases covered:
- SAFOX 11 AF System is already good down to -3EV
- Weather Sealing is obviously no problem for Pentax
- The K-3 already has Dual SD Card slots so that shouldn't be an issue
- Since Pentax utilizes Sony sensors, sourcing a 24 MP sensor would be easy
- The 645Z has a tiltiing screen so obviously that wouldn't be too difficult for Pentax either
The only real challenges for Pentax would be improving their video mode (focus peaking during recording please!!!!), integrating Wi-Fi into the body, possibly the issue of stabilizing the larger sensor, and of course the lenses. Honestly, I don't think it would be difficult for Pentax to better the D750.