Local domain whitelist? Would that work?

Created by the user? Like, who's going to go out of their way to create a whitelist of associated websites so it's okay to share across those sites?

So, I'm totally on board for this! The only other thing would be to just not let cross domain sharing. at. all.

Which of course wouldn't be abused. Either set willy-nilly, or companies purposefully implementing broken methods to get an exemption so their tracking cookies work /s