From what I see on my CHDK'ed SD800, a color preview image is no problem as long as the clipping and focus indicators blink. It makes also much easier to guess which channel is blown.
The focus mask could blink at a different frequency to make it more distinct of the clipping masks.
For the histogram, of course it shall be raw based (how could one possibly imagine it else?) but it could be nice to have it outside of the frame, to allow easier composing/framing.
Ditto for the exposure parameters (and a few other optional indicators, as a flash-ready icon or the # of shots left in a burst and/or on the card, could be useful too).
Hard decision as EVF real estate is quite scarce, but ideally I'd say that only image-based indicators (clipping and focus masks) do belong to the image area and others should go in the margins.
And then, there still is a big question : even if we've put the WB out of our way (but I also see and dream in colors
), how to render the tonalities in the preview image?
I really like the ground glass, among other reasons because I can see into the shadows with it (and maybe more generally because the scene is prettier than a default rendering). Some automatic fill light could be useful there... It's actually not a problem if the preview is not that pretty as long as information (ie the detail recorded in shadows and highlights) is readable. Btw, that would make me prefer a brightness correlated to the exposure parameters.