I am a horrible person and you people are horrible for encouraging me.
David | Prince Charming: Shall be having nightmares of his wife not remembering him, or her dying, or Emma and Henry dying and he can't do anything to stop it.
Howard Stark: Is going to dream of the atomic bomb, World War II, Hydra, Nazis, and wandering around a frozen wasteland, never finding the best man he's ever known.
And now for the worst ...
Castiel: Doesn't sleep - BUT he is going to get knocked on his ass by a Nightmare, and that's going to knock him out, and make sleepwalk. Where he'll relive the nightmare of being God!Cas and Levithan!Cas, all in one. Yaaaaaaaaaah!