Sounds like you should definitely write the fic! There's a fascinating difference in the worldviews of Buffy and Supernatural. Like all Joss Whedon's shows, Buffy is very interested in chosen family, and in the experience of people living on "the raggedy edge", as Mal puts it in Firefly, who nevertheless build a community that's worth preserving.

Supernatural almost aggressively resists the creation of a chosen family/community--the Roadhouse burns after a single season, Bobby's house burns eventually, and secondary characters consistently die rather than accumulate into a band of "Scoobies." The Campbells were amoral and the Men of Letters exist only as a memory. We're told that a larger hunting community is out there, but Dean and Sam never really plug into it, and when other hunters do turn up they're enemies about as often as they're friends. Cas ends up with the Winchesters because his own people betrayed him, and when he tries to build a new coalition of angels who share his values, he betrays them.

I'm not really complaining, even though I've often wished that SPN would expand its universe to include more side characters, because I do think that it's a more or less deliberate choice. The universe of SPN is fundamentally bleak: you have free will, and maybe in rare moments you can break away from the tracks laid out for you with a strong enough exercise of force, but for the most part you're doomed by your own nature. It's not morally black and white because the show recognizes that characters like Lenore and Benny are tragic, but it also insists that in the end the only choice they can make is whether to die or kill. And I think the show applies that same logic to Madison: I found that decision incredibly frustrating because I don't buy that there's no practical way to tie someone up three nights a month, but from the perspective of the show, "the other shoe always drops." Even Sam didn't avoid his destiny: in the end of he drank a bunch of demon blood and got possessed by Lucifer in Detroit, just as predicted. It was just that Lucifer misunderstood the prophecy. The SPN-verse is almost a tragedy in the Greek sense, and I appreciate that even though I don't agree with it. It's so different from the worldview presented anywhere else on television.

Anyway, I think it would be interesting to drop Sam into the middle of a different paradigm. I do think that if he felt like he had a genuine choice about hunting he might eventually come around to the idea that it was something he wanted to do, rather than just a bitter necessity. And it would blow his mind to be around people who could know the truth about him and still accept him--SPN asserts pretty strongly that you can either be honest about yourself or be loved, but not both, at least outside of the circle of Sam, Dean, Cas, and Bobby. Lisa, Amelia, Jess: you can only have them, and the normality they offer, at the price of being someone you're not.

Re: Pairings

I mostly read D/C, Sam/Cas, and Dean/Sam/Cas, although I read Sam/Dean when it's a good author or an interesting idea. I've got a bit of a dark!ship for Dean/Alastair (yay, Stockholm Syndrome! IDEK), and I wish there were more Sam/Ruby. I'd like to try writing it some day. Btw, if you haven't checked out Obstinatrix's fic and Badbastion's art for Sam/Cas, Dean/Sam, and Dean/Sam/Cas, you're missing out.

As a side note, have you considered signing up for Summergen? It's a good chance to write some gen Dean and Sam and meet more LJ folks.