I feel like they've kind of been foreshadowing it the same way they did in S9, filming it so Dean's eyes are dark/black/in shadow, etc.

Yes, and he's started staring angstily into mirrors again -- can't be a good sign. ;)

Though I do wonder, what with Cain's whole vigilante act/how he was rationalizing his kills via an appeal to the greater good, if maybe that's foreshadowing something a bit different. Maybe it's not being transformed that will ultimately lead Dean to the dark side, just the job and the 'good' fight.

I think it was frozen_delight who had the devil's trap idea, and it's really intriguing. More interesting than mine, which was just Cain saying something potentially important as he died. ;)