Halloween episodes are a staple of modern cartoons. Ever since the first Simpsons Treehouse of Horror, all sorts of great shows have put out great holiday-themed episodes. The DCAU rarely partakes, though. There was that Christmas episode of Justice League Unlimited, a couple of other Halloween episodes here and there, but it’s never been a regular thing. “Trick or Threat” is the kind of story that makes you wonder why they don’t do it more often. It’s the great idea that makes you say, “Why didn’t I think of that?” We open on a spooky moonlit night. Packs of munchkins