Pretty much any team that doesn't already have a player of Cousins' caliber will be prepared to offer a boatload of assets for him.
If the Kings are interested in winning now rather than later, I would expect them to prioritize packages that include players that can help them win now over packages centered around draft picks.
For example, if the Wizards offered Nene (expiring), Bradley Beal, Otto Porter, and Ramon Sessions, plus a couple future picks, for Cousins, Collison, and Caron Butler, isn't that really attractive deal for the Kings?
Reuniting Wall and Cousins could be fantastic for Washington, and they could still create space for Durant.
With Cousins my concern is that I think he needs to go somewhere that already has a player in the locker room that has the resume and the personality to tell Cousins to calm down and not talk so much. He needs somebody else to be the alpha dog, somebody with the talent and track record of winning that Cousins will respect. For that reason, I'm not sure Boston is the best place for him to go.