One feature I'd really like to see is "Composition Mode" maube replacing the Print button I guess. Composition mode would be where you shoot and are able to review the pics, and then decide if you want to keep them or not, it would maybe have a buffer of maybe 3-5 shots.
I shoot a lot tethered using C1 Pro and always use the "Composition Mode" to get lighting values correct, before actually saving images, this enables you to shoot and check the image, adjust, get everything dialed in, then switch to normal shooting once everything is setup and looking good.
It may be a long shot and something not considered before, but it would be a great feature. I realise it is easy to just scroll back and delete shots in camera, but sometimes this can slip the mind and confuse you when you dow nload and view the images. Having an easy to access switchable mode would be great, and with the option of keeping the last few shots and writing them to the card would be fairly simple I'd imagine.
Other than that, looking forward to a new release of the 5D, I can't see it being even close to the 40D, its gotta sit pretty close to the 1D3S in specs, regardless of price, as it did when it was first released, bring it.