My only experience is with Phase backs, so my comments are relative to them.
First, provide a big, beautiful, hi-rez screen (like the one on Nikon's new D3) as well as support for untethered, zoomable, live view. Ideal for focus confirmation, and in the case of techincal cameras, perfect for composition and focussing.
Second, provide a user programmable function button, or two even. I'd use it (or them) to quickly put the back in custom white balance mode (hold an expo disk in front of the lens, press the function button and take the WB shot and your done), or to wake up the back (really only needed on technical cameras).
Third, for use with techincal cameras, a bluetooth sync input, along with a small bluetooth transmitter that plugs into a lenses sync output socket, and a keyfob-like device that can be pressed to wake up the back. Again, this is really only applicable to Phase backs (and I guess Mamiya backs) when used on technical cameras.
Fourth, the abililty to load different user tone curves would be nice.