- newer sensor, more megapixels but more importantly, more dynamic range.
- 14-bit RAW or better
- weather/dust sealing
- same buttons as X-Pro1 or bigger, move away from small XT-1 buttons
- more responsive all around, AF, OVF/EVF, card writes, etc. to live up to its pro name
- high-speed electronic shutter up to 1/32000 of a second
- Q-menu deeper customization options, let each custom bank feel like a different camera for varied shooting scenarios, allow settings to be exported to card to set up on multiple bodies
- richer flash controls & flash synch, license Canon's flash tech and/or work with lighting manufacturers to bring off camera high-speed sync
- interval timer shooting
- if it has Wifi and other bells & whistles then have a classic/retro option to turn off everything but basic still camera functions to prolong battery life
- make a monochrome version, ideally make the sensor unit modular to swap out