Don't tracking cookies work because cookies are forwarded across domains? If so, what would be the consequence of browsers only sending cookies to the domain that 'owns' them in the first place? If there is some reasonable functionality, perhaps these should be regarded as opt-in, for example: "foobar.com is requesting access to cookies from ads.google.com. foobar.com says it uses these cookies to enable feature X. Allow access? <Y/N>".