I've jumped on the Pentax ship since I didn't really need a back-style camera for tech cam use, but a friend of mine does, and we both agree on many features for a potential future back.
I'll list the points here since a survey likely won't convey this information.
- sensor as close to full 645 size as possible, but more than 50mp is overkill for most things, work on shallow incident light performance for tech users instead.
- definitely CMOS, because of real-time liveview magnification, it's what kept me from getting into digital backs years past.
- tilt or maybe even a swivel screen, I didn't know how much I needed one until recently. phone/tablet tethering isn't a solution.
- it would be cool if the back had a HD-SDI or HDMI port to allow users to connect their own HD monitors to the back.
- low-latency performance. Waiting an eternity for image preview to show up on the 645D is why I never bought into the system before.
- open source software platform to allow for custom firmware, additions and tweaks by the community - think Magic Lantern for the Canon cameras but as the official driver.
- a big battery.
If this theoretical back were available mid last year and cost less than $10K, I may have not have had the 645Z right now.