It would be nice if they supported paid upgrades. For example, if someone has an inexpensive (but not free) Lite version and then a more expensive full version, it would be nice if you could buy the Lite version and then get a discount on the full version.
The same idea applies to the case where a developer decides that they have made enough changes to an app to post it as an entirely new one (rather than allowing free updates). It would be nice if they had the option of giving owners of a previous version a discount.