The simple solution is to forbid 3rd party cookies (and while we're at it third party JS as well, which I think is a much bigger problem than 3rd party cookies. I'm sure that will send shudders through the industry). And enforce it at the browser level by default and put up a big fat warning what the consequences are when you disable it.

That way we don't need to have silly laws that nobody will respect and we can all get on with making stuff work.

Third party JS opens so many cans of worms that I think it would be better if we just forgot about that whole idea, it'll never be secure and it puts too many juicy bits in the wrong hands.