Duuuuude do you have any idea how much time I spend thinking about this?
Man, it feels weird getting asked this out of the blue -- I didn't really have time to think up a proper list. @_@ But yeah there've been a bunch of fandoms that've crossed my mind that I never considered trying to introduce to the RP because I either thought I couldn't play them well enough or I wouldn't have time.
Just a few off the top of my head:
Death Note and Bakuman (I see these two as brothers)
Avatar: The Last Airbender and Legend of Korra (I see these two as sisters)
Digimon
Animorphs
Torchwood
Pretty much anyone from the Beatles' extended family, but especially the Wings.
Firefly
More to come soon. I just need more time to think about this. There are tons of fandom ideas that have gone through my head since I started this RP and I know I'm forgetting a bunch of them right now.