Warning: spoilers ahead for the second episode of Supernatural Season 13, called "The Rising Sun."
Sam and Dean Winchester currently have their hands very full as they must figure out how to deal with having a nephilim around at all times. As the son of Lucifer and Kelly, Jack is being hunted by angels and demons, and the Winchester boys can't quite agree on whether to protect him or figure out how to kill him. "The Rising Sun" was pretty heavy as Dean argued that Jack is going to embrace his evil side no matter what Sam tries to do to save him. There was some levity in the episode, however, as Jack began to experiment with modern amenities, and it was in one of those lighter moments that we may have gotten a big clue about how the Scooby-Doo crossover is going to work.
After the boys checked into a motel to take a break from all the driving, Jack started playing with the TV, and he discovered an episode of Scooby-Doo. He was clearly interested in the show, but Dean switched off the TV before Jack could see much of it. The rest of the episode was spent exploring all of Jack's various powers as the son of an archangel, and the Winchester brothers were constantly being surprised by what Jack could do. Even Jack was discovering abilities that he didn't know he had. Now that he's seen Scooby-Doo and shown interest in Scooby-Doo, doesn't it stand to reason that the Scooby-Doo crossover will happen when Jack somehow uses his abilities to zap the boys and (somehow) Castiel into animation?
If that's the case, then the Supernatural team has been planning this Jack story for quite some time. The actors recorded for the episode a while ago, and there was already some art completed back when the Scooby-Doo crossover was announced back in May. At that point, my money was on the Winchesters going animated happening as the result of hallucinations or dreams or a new Trickster in the mix to mess with them. Supernatural has gotten plenty weird before; a Scooby-Doo crossover as a case of the week wasn't out of the realm of possibility. Now, the odds seem pretty decent that Jack's interest in Scooby-Doo in "The Rising Sun" was a hint of what's to come.
Then again, Jack's interest in Scooby-Doo may have just been a nod to fans who have been waiting for the animated episode since May. This could have simply been an Easter egg of sorts to remind us of what's to come. It's entirely possible that the Winchesters will go animated because of dreams or hallucinations or a mischievous supernatural being. We'll have to wait to see for sure.