An idea for a Supernatural/Smallville crossover wasn't exactly what you might expect.
Supernaturalis currently enjoying its 12th season, and there's no end in sight. Smallville, the show that arguably made the CW into the genre show haven it has since become (and perhaps helped make Supernaturalpossible), took its final bow at the end of its 10th season.
Now imagine, if you will, a crossover between the two shows. It's easy if you try (sorry), especially given how crossover happy the CW is with their current crop of superhero TV shows. And why shouldn't they be? They're absolutely killing it in the ratings. But a Supernatural/Smallvillecrossover would have come at a time when such exercises weren't necessarily a slam dunk.
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Now, keep in mind we're not talking about something as basic as the Winchester boys making a pitstop in Smallville, Kansas and teaming up with Clark and friends to take on some supernatural menaces (although, the idea of introducing a DC character like, say, the Phantom Stranger that way would have been kinda cool, but I digress). No, according to Beyond the Gates director Jackson Stewart, who once worked as a production assistant on Supernatural, it would have been something a bit more meta.
“There was a lot of really cool ideas that some of the writers had,” Mr. Stewart told Entertainment Weekly's Entertainment Weirdly Sirius XM show. “They had an episode where they really wanted to do a Superman curse that was about every actor who plays Superman ends up getting killed. So they find out that Tom Welling from Smallville is next and have to save him. I thought that was one of the funniest ideas that I’d heard.”
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The "Superman curse" being alluded to here is the urban legend that anyone who wears the red cape is bound to have some horrible personal or professional fate befall them. It stems both from actors' fear of being typecast after playing the character, and the tragic endings met by TV Superman George Reeves (who killed himself after a long battle with depression) and Christopher Reeve (who was paralyzed in a horseback riding accident). From a personal standpoint, I'm not sure this concept would have been in particularly great taste, although I have to give them credit for thinking about this in terms of something other than a pure character team-up.