Destiel, Sabriel (sam and Gabriel) and Wincest are the most popular slash pairings but as for heterosexual onea, I dunno except Dean/OC or Sam/OC or any other major reoccurring characters being paired up with ocs.
I like the Trickster Gabriel myself. I have the first 8 seasons of Supernatural and am waiting for season 9 to be released on dvd.
I have a two shorts for Supernatural, but as for the ships, it's always up to you. Though I've never seen any Bobby/Sam/Dean, but if that's what you like I say go for it!