Usually we have the requierement to isolate SharePoint sites from specific Business Lines. This means no allow the Site Owners to give permissions or Share content with some AD groups.

Currently peoplepicker filter is only possible at Web Application level (ActiveDirectoryCustomFilter), but it means we are forced to create different URL's to have an effective segregation (no peoplepicker search results --> No permissions / share given).

To be able to manage an effective audience segregation without having multiple URL's (Web Applications), it should be great to be able to filter peoplepicker at Site Collection Level