Cataloging features that let Pro users keep track of large slide inventories would be well appreciated. I know more than a few professional photographers that have libraries of tens of thousands of slides taken over their careers. If there would be a way to tag each image with "storage location" and be able to sort by that feature, that would be nice. You can probably do that now with Keywords, but having a real feature (with more specific design) would be nice. Since professionals also might have several slides "checked out" of their collection at any given time (such as being sent to a magazine for use in an issue) getting the PhotoPro to keep track of this (either by putting a little flag on the photos that are out or by making them fade out a bit) would be nice. PhotoPro could integrate with Address Book and iCal sort of like how Delicious Library does.
These features would appeal to the hardcore photographer with massive numbers of photos. Actually getting all those images into the computer and into PhotoPro would still be a major task, but one the professional could pawn off on a summer intern.