I'd love to be able to use the DoF Preview button to switch to APS-C crop - preferably while holding down the button (though I could make do with a toggle). When shooting sports, I shoot an occasional distant shot that needs a lot of cropping, and it would be great to be able to "zoom" in and pre-crop it on the fly. This would make the Internet Proofs much more sellable to my customers (even though I crop them anyway after purchase, I use very small images for the online proofs).
(And if you have time left after that and can add a 14 bit global shutter via firmware so I can use my studio strobes at 1/8000th, that would be fab, too!)