I guess it's becomming a bit of a dead horse these days, but I would like Canon to introduce a propper TAv mode where the variable is ISO. Pentax can do it so why not Canon? Instead of this not very well implemented Auto ISO in manual, where there is no EC available. I could see using a propper TAv mode a lot. Of course when the current exposure modes that Canon offer us were devloped although the ISO/ASA values could be changed it was not a variable value on the exposure triangle. Since we have had film on rolls with multiple frames avilable Film speed has effectivly been a fixed value for each roll of film. So you could treat it as being a fixed value for your exposure system as you could only changed it every 12/24/36 exposures or whatever. Now of course it is possible to vary the snsitvity for every shot, theorertically by any amount you like, like some film cameras did with shutter speed in the early days of electronic shutters. The Pentax ME Super did it in Av mode for example (a camera I owned in my youth). So it really should now have it's own mode.
Talking of exposure related modes I would like to be able to set an EV in manual and lock that, so that with a roll of the front wheel you cange both shutter speed and aperture at the fixed EV. This would be like "Program Shift" for manual mode. Using P is a problem in the current cameras as you are tied to the cameras metering system, although with EC. It also changes back to the "default" choice every time you press the shutter.
Mostly at the moment I use Av for general shooting and M for most stuff where Av won't do what I want it too.