Idea Summary
If you have a lot of regions on Page 0 it becomes a mess using the Application Builder to work on your pages. Because every region is being shown and it's hard to find the region you're looking for. 

Use Case
Ideally I just wanna see the regions that are being rendered on the specific page. Sometimes it depends on the user role, in this case it's not possible to distinguish. 

Preferred Solution
If possible, only the regions being rendered should be shown in the Application Builder.