In kind of a tangent, in response to the premise of bdotp, I have to reminisce about the Bodie/Doyle (Professionals, from '80s-era fandom) fanfic. One subgenre picked up the actors' other roles (either one alone, or in a couple of cases together) in other shows, and crossed them with Professionals so that Doyle, instead of slashing with (um, new grammar for old tricks!) Bodie proper, slashed with Bodie's identical cousin from an obscure thriller movie. This often segued into Bodie proper noticing that *he* always wanted to do that, and led to intra-show slash, and thus was order restored to the universe.

I'd *like* to say that slash pairings (with a few exceptions in recent years) are *all* fanonical, but the point of slash in many cases is that the only difference between slash and the show is that the guys are spending 24 hours a day together, instead of 16 or more. That is, are Kirk/Spock or Arthur/Merlin not partners in nearly every sense already? But there's certainly a point at which finding slash can be fanonical rather than a foregone conclusion. And it's fun.