This scene just popped into my head; the background I have is that Dean was sent to Hogwarts after being expelled from illvermony, Harry is head auror and Dean was a Slytherin in the year above the trio once he transferred.

Sam is not magical and Dean cast the killing curse in self defence. Harry has asked Dean to cast a patronus, a spell he’s never mastered before, as the magical community uses it as a basis for determining the state of a wizard’s soul.
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“The world isn’t split into gryffindors and death eaters. It’s about time we really started moving on.” Harry seemed to have done a lot of growing up since Dean had last spent any real time around him at school.

“When you’re ready to run for election as supreme ruler of the world give me a call,” Dean answered with a smirk, “I’ll vote for you.”

“I don’t think so, I’m happy here in my office making sure no one else tries for the job either.” He hesitated. “Are you stalling?” Dean shrugged.

“Just taking time to think.” He concurred softly. The truth was he was wracking his brains and he was struggling to come up with a memory that wasn’t bittersweet in some way. Eventually he hit on something.

“Alright,” Harry dimmed the lights with a murmured spell so they could see the form of whatever Dean’s patronus might be, “expecto patronus.” Dean hadn’t expected for to work. He’d cast a killings curse the day before and never managed this spell before… So when something exploded out of his wand with the force of small train he nearly fell over in shock.

“Whoa!” Even Sam was impressed judging by his stunned silence. Harry looked like a kid on Christmas. “What is that?” Dean stared at it. Something with hooves and antlers but they weren’t like any antlers he’d ever seen.

“Some kind of deer?” Harry offered having never seen it either. Sam burst out laughing when he realised what it was.

“It’s an impala!” Dean grinned almost as much at sam’s laughter as the situation.

“Well, that was completely expected.” He deadpanned after a minute. Harry still looked confused.