It’s Not A Déjà-vu, It’s The Time Warp by pforte
Merlin/Torchwood/Harry Potter ; Merlin/Arthur, Jack/Ianto, Remus/Sirius ; R
"We’re secondary characters in a children’s fantasy series. Remus doesn’t like the name but we’re calling this little group PISSA. Means Popular and Ill-treated Slash Ships Anonymous."
"A self-help group for the unluckily slashed," Remus added helpfully.
If you're a fan of any of the three ships featured here, you'll love this. If you're a fan/familiar with all three, it's like hitting the Jackpot of Awesome. pforte nails it - the fanon cliches, the ridiculousness, the way all three ships are exactly the same when it comes to badfic, and the whole thing is so wry and well-done and comedy gold. Merlin gets possessed by a self-insert! Sirius is made to kiss girls! Jack's tired of re-enacting bad gay porn! Ahahahaha, pforte wins everything, the end.