Hmm, that's a tough one since I never seem to write pairings.
Transformers...can't really think of any. There were canon couples - Optimus Prime and Elita One, Ironhide and Chromia, Firestar and Inferno, Spike and Carly, Arcee and Springer - but I'm not particularly interested in writing any of them.
For Supernatural, Dean & Castiel. I'm quite convinced that Castiel is utterly in love with Dean, but I don't think it's sexual (I read angels as asexual (generally, yes, I see you there, Gabriel) but that doesn't necessarily equal aromantic) and I am certain he has no idea what to do about it. I guess Sam and Jessica and John and Mary Winchester since they're canon? For a show with so many chicks of the week there aren't a lot I'd want to come back to. I would have liked to have seen more of Sarah from the creepy painting episode in season 1 though.
Lord of the Rings is another one where I'm kind of coming up blank. I do like fic with Elrond and Celebrian, and I'm definitely a fan of Erestor and Glorfindel. I like Celeborn and I'm not as sold on Galadriel but I do like her in the hands of the right writer.
In the Doctrine of Labyrinths we have a number of canon couples but none of them seem to end well. Sometimes that just means a breakup and sometimes it means horrible tragedy. I wrote To Chain Together about the aftermath of the end of Felix and Shannon's relationship, as canon didn't quite satisfy me. There's also Mildmay and Mehitabel (breakup), Felix and Gideon (horrible tragedy), and Felix and Malkar (nightmarishly abusive). Felix and Mildmay (incest) was more than hinted at through most of the series but I'm not sure it applies by the end, if that makes sense without spoiling the books for you.
The Iron Elves didn't really have couples. Konowa wound up with someone, and his parents were in the novels but one of them was a squirrel at the time.
Yeah. It makes sense in context.