An RBL-like blacklist of high-traffic sites that frequently link to other sites without asking permission first, which could be used to build a Referer blocking list. The idea is to encourage sites like Slashdot to start asking sites they link to, else they simply won't be able to link to anyone using the list.
I like the concept here, but it'd be extremely difficult to police in the same way that the mail RBLs are. The easiest way might be to have a code of conduct for high-bandwidth sites to follow, and run the RBL like rfc-ignorant.