I'd love to see the ability to utilize multiple partitions/disks when defragging. My largest partitions are nearly always nearly full (under 5% free), but the smaller ones are about half full only (~50% free). The free space on the bigger ones is hardly sufficient for effective defrag, but by utilizing the free space on the smaller ones it could be done -- and faster too.
Thus far I've never come across this kind of feature, but it would certainly seem logical.