As a fan of Supernatural, and of geek genre fiction in general, I thought I would offer my advice to any struggling screenwriters who might want to write a spec for Supernatural. I don't have time to write one myself, with my trying to get young people to vote and all. While my wife might attempt one, it would be based one one of the several discussions we have had over ideas. I just want to make sure that whoever ends up writing on Supernatural continues to entertain me. See, it's a selfish reason.
[Before I continue in this vein though, I would like to direct you to Gaze Theory's post regarding Unesco allowing its members to ban US film and music, something that could have serious affect on the entertainment industry.]
Okay, now back to the advice:
First, select an Urban Myth with supernatural elements and have that be the basis of the A storyline. If you don't know any you can go to Snopes to find one to research. You can also take a non-supernatural urban myth and make it "mysterious" or take a standard horror tale and modernize it.
Second, make sure to include the trope of "secrets." Every episode has had reference to how secrets come back to haunt you. This includes the fact that the brother's are ghost hunters being a secret (see below).
Third, have a small romantic element. By small, I mean very small, and usually involving Dean because Sam has his own C story going on (see below).
Fourth, remember the "secrets and lies" comment above. Well the brothers have a great longterm conflict building due to their dishonesty. In almost every episode the brothers have claimed to be law enforcement and have been found out. In the Pilot, Dean was arrested for impersonating an officer. In Phantom Traveler they potentially p.o.'ed the Feds by posing as Homeland Security, and in Skin the doppelganger committed murders while looking like Dean. Sure the Winchester brothers killed the doppelganger, thus the case is closed, but Dean Winchester had an APB issued for his arrest for murder and he probably has a warrant from when he broke out of jail in the Pilot. They are building up with this and if you pick up on that, it will look good.
Fifth, speaking of building up...remember that C storyline with Sam I was talking about. We all know that he saw his girlfriend murdered in the Pilot, but he also saw her ghost (in daylight and dressed in white which is a possible reference to the Lady in White in the Pilot). Try to speculate what her supernatural yearning/need might be, and remember their mom was killed the same way and may be lurking around somewhere as well.
Sixth, don't try to trick the audience. The narrative should have surprises, yes, but they should be conventional genre surprises.
Seventh, remember that the brothers are the exception to your typical horror movie rule. The brothers know from moment one that they live in a horror show. They may not know if they are in the middle of a horror narrative at any given moment, but they know what is going on. Remember that Sam figured out that the doppelganger was posing as Dean very easily because he was expecting something to be wrong.
Those are my thoughts...oh, and I might be happy to read any specs you might be writing and comment back.