For your needs D800 is way too low in megapixels.
You might try an H4D-200...http://www.hasselblad.com/products/h-system/h4d-200ms.aspx
...but I think that neither Hassy will satisfy you
Seriously, tough, I think that at this moment
on 5D Mk III we
(Canon + community) should concentrate in finding concrete ways to improve it
. And I think that the areas are:
- better SNR from sensor
(I dunno if this is possible anymore via firmware - but definitely they should try this)
- better customization of camera
(imho, they should put more options on more buttons - there are enough options which are available only on one buttons and other buttons are for many just "waste of space")
- ...perhaps they must do something to make the life of camera programmers (Magic Lantern, CHDK etc.) easier?
(there are enough ways to do this but first I want to see what's your opinion)
Any other improvement?...