This was not a rumor. The Yankees and Red Sox GMs/owners actually agreed to this trade one night. The next morning they woke up, realized what an insane idea it was, and mutually agreed to nullify it. Point being, DiMaggio was too closely associated with the Yankees and Williams with the Red Sox. And the same reasoning applies here. This will never happen.
It doesn't matter how closely Harper is associated with the Nats franchise. We will see how loyal he is when the Yankees and all the press clippings he can handle come calling.