I want to be able to set shutter speed faster than 1/250 in Av
You mean with non-HS flash modes? If so, that's a hardware restriction...
Some more control over auto-ISO would be nice, but most of all I wish that they would include an intervalometer (like used to be included in the G-series, until marketing decreed that it should require the purchase of an ungainly and awkward external trigger...).
Overall though, I think the 5DIII firmware is pretty good - they removed a lot of idiotic restrictions on the 5DII (like bracketing options), and it is rare that I find myself needing something it does not do.