[Still Thinking]

Advertisers will just demand that web sites using their ads install gateway software to make their cookies appear local, along with software that will block displaying content if ads are blocked.

Which the browser makers could hopefully defeat by pretending to accept them and/or accepting them for the duration of browsing that site and then delete them, which would limit their snoop value.

[SauronOfMordor]

It will be an arms race between ad blockers, and ad makers who will try to make it hard for software to distinguish ads from content.