I've never really written any mad pairings, but I've read some great ones.
Oh God, actually, not great. I just remembered the Hermione/Snape fic I somehow ended up reading once and I am eternally scarred.
I once read a fic that had all of these pairings in:
Ryan/Spencer, Ryan/Brendon, Ryan/Jon, Ryan/Brendon/Jon, Ryan/Amanda Palmer, Ryan/Lindsey(Way), Ryan/Gerard, Ryan/Zack(Hall), Ryan/Pete Wentz, Ryan/Mikey and Ryan/Chris(Carrabba).
You should have seen the warning list
I read this fic that was Gerard paired with this pop singer from some foreign country and I didn't have a clue who he was. That one was weird.
There's an MCR gsf somewhere but it
includes Worm and it was so freaky. And then I once read a Bob Bryar/Brian Schechter (or however it's spelt) fic
with tentacles.
But my all time favourite was a prison fic with basically every musician from every genre included and it was insane. It went from Kitty/Lynz which is fine by me to Jimmy Urine and a country singer who was the guy that locked the cell doors every night. You want crack? Read that. If you can find it anywhere because I lost it.