How much can Apple ACTUALLY change iOS' UI?

The home screen being mainly an assortment of icons from other developers (or their own), the only way Apple can change their UI design is through their own core apps. That, and the home screen (which I really hope they change).

If they, like many people say, change iOS' UI "radically, that would mean that they have to rearrange the home screen in some way, and in a way that doesn't need developers' immediate changes (ex. if they make smaller icon requirements, developers need to create smaller icons for better results, and not every developer can/will do that right away, causing visual fragmentation). So, how else would they change the homescreen? More whitespace? Widgets? Any thoughts on what Apple might ACTUALLY change (by logic) in iOS7, UI-wise?