I compiled this list because Gojyo and I were talking about “canons that could cross over,” and because Hikaru recognized Sakuraba. The ones that first came to mind were ones like Ouran, HanaKimi, and CLAMP Campus Detectives. But if you give a mouse CLAMP Campus Detectives he’s going to want to cross over with X-1999, and why the hell not? So the list extended from “canons that take place in our (modern, more or less) world” to “canons where magic is kept underground, and could thus take place in our world.” That brought in canons like Bleach, Buffy, and Dead Like Me.
Now I have a rough list of canons which theoretically take place in our world, more or less now. Why? Because you might know the same noodle joint in Tokyo, or be long lost cousins eight times removed. Please tell me if there are canons that I’ve mistakenly added or missed on this list.
Note: DC and Marvel comics are not included because, so far as I know, people know that Superman exists in them. Because superpowers are commonly known to people, they didn’t fit into the “could be our world” quite precisely… but I don’t know comics canons. Hit me up about that!
Note II: Starred canons don’t really take place in our world, but they have some connection to it.
PLEASE COMMENT AND LET ME KNOW IF CANONS ARE ON THE LIST WHEN THEY SHOULDN'T BE, OR AREN'T ON THE LIST WHEN THEY SHOULD BE.
Achewood
The Addams Family
Arrested Development
Azumanga Daioh
Beavis & Butt-head
Bleach
Buffy the Vampire Slayer
Cafe Kichijoji de
Clamp School Detectives
Clueless
Coraline
Dead Like Me
Donut Girls
Due South
Earthbound
Elf Only Inn
Excel Saga
Eyeshield 21
Fables
FAKE
Fruits Basket
Full Metal Panic
Fushigi Yuugi*
GANTZ
Good Omens
Grey’s Anatomy
HanaKimi
Harry Potter
He is my Master
Hikaru no Go
His Dark Materials Trilogy*
House
Kyou Kara Maou*
The Law of Ueki
Lawful Drug
Lupin III
Mahou Sensei Negima!
The Melancholy of Suzumiya Haruhi
My Life in Blue
Ouran High School Host Club
Pani Poni Dash!
Prince of Tennis
Ranma 1/2
Sailormoon
Sandman
Silent Hill 3
Smallville
Tsukihime
Veronica Mars
Wedding Peach
Weiss Kreuz
Wild Adapter
X/1999
xxxHolic
Yami no Matsuei
Young Wizards
Yuu Yuu Hakusho
Zatch Bell
This second list is, so far as I know, the list of canons which are attached to our world - either in the past or present. This is where, say, Les Miserables comes in, as do post-apocalyptic series. I think it’s impossible for SEED and Wing to exist at the same time, though... Again, correct me if I’m mistaken.
.hack//SIGN
Airgear
Aladdin
D. Gray-man
Dynasty Warriors
Firefly
Fullmetal Alchemist*
Gankutsuou
Gundam SEED Destiny
Gundam Wing
Kami-sama no Ude no Naka
Les Miserables
Neon Genesis Evangelion
Outlaw Star
Peter Pan
Pirates of the Caribbean
Runaways
Saiyuki
Soukyuu no Fafner
Star Trek
Star Wars*
Tenchi Muyo!
That 70s Show
Trigun
Wraeththu
Well, I hope that was interesting for everyone!
Edit(s): Loveless removed (ears)
Runaways (Marvel)
Devil May Cry (demon Jesus)
R.O.D. (wtf Britain?)