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?...