A useful feature would be to have the opposite of what the current Zonereferrers feature does.
Domains would not be blocked by default, and you can add domains that you do wish to block.
The rationale behind this is as follows:
You may not wish to block most other websites including image search engines, for example they might feature you in a harmless article that brings you valuable traffic, so most of the time you don’t mind actually footing the minor bill for the viewership. In this case, it’s impossible to keep up with the list of domains you’d like to whitelist.
You only wish to block one or more specific domains if and when you find them being deliberately abusive.
With this in mind, this “reversed” Zonereferrers feature is arguably more important and useful than the existing version.