This lens must be Canon's worst kept sectret. We all know it's coming, but no one knows when.
Optics, there's probably a marginal increase in sharpness, CA control etc. A true 24mm would be nice though. My copy seems to come in around 25.5mm. I don't think there are many gains to be made optically.
Build, hopefully Canon will make this lens a bit less industrial. Sure the current version is practically bullet proof, but it's very heavy and bulky for what it does.
Other Stuff, An IS unit would be nice, but not essential, Nano coating is nice for flare control...not that the current version fares too badly. The current AF unit is very fast and accurate. I seem to be one of the few guys that likes the reverse lens extender, especially with the hood.
Price, we all know that a new version of this lens is going to be really expensive.