What kind of improvements do you think the 5DIII needs?
Well, as you see I don't own one (yet, still waiting for the price to drop or if it doesn't to get a used 5d2). But this is what can be improved w/o copying features from the 1d like spot metering on the af point:
* pop-up flash and/or built-in rf controller vs attaching a bulky dedicated controller or 580ex-type flash just to have master capability when only using external flashes. I'd be absolutely ok with a hole in metal casing like on the 6d, in fact when they're at it they should build in wifi, too.
* better servo af indicators (instead of the semi-random blinking, if that feature ever makes it to the 5d3). Of course my 60d (and the 7d afaik) don't have that either, but I certainly won't try to track sports with it.
* faster sd controller - as it is sd is only good for jpeg backup and starting magic lantern [EDIT: remove "crippled" because I don't think the slow sd slot is intentional either]
* faster x-sync
* faster contrast af (like the 650d af pixels)
* replaceable viewing screens
* faster lock with af assist beam
* better af with non-f2.8 lenses (like my 70-300L that only uses non-cross on the sides even when at f4)
* well, for me: swivel screen (yes-this-is-very-useful)