A Word On Crossovers
For example, the easiest points in each fandom that can be used to make a crossover include:
Time-and Space Traveling - You can get there from here or now.
Watchmen: Dr Manhattan
Doctor Who: The Tardis
Terminator: The Sarah Connor Chronicles: The transporter
Heroes: Hiro ; Immortality
Supernatural: The Genie
Buffy the Vampire Slayer / Angel: Portals ; Unnamed "Normal Again" Demon ; Immortality
Life on Mars: hand-waving cars and bullets
Medicine - We'll take you there with flashing lights.
House
Grey's Anatomy
ER
Scrubs
Lost - Jack
Heroes - Peter, Emma
Criminal Activity - You have the right to remain silent.
Castle
CSI
Bones
Supernatural - Even if they're fake IDs.
Heroes - The FBI, The Company, Matt Parkman...
Angel - Angel Investigations
Human Target
Batman
La Femme Nikita
Life on Mars / Ashes to Ashes - Character's original timeline also viable.
X Men
Undercover - We're there even if you don't know it.
Human Target
Supernatural
Buffy, the Vampire Slayer
Angel
Heroes
Human Target
La Femme Nikita
Dollhouse - Not only is the house undercover, but the Dolls could be anyone.
Lost - Character backstories
X Men
Harry Potter
Good Omens
Watchmen - Within timeline constraints.
Batman
The Supernatural - We know weird.
Buffy, the Vampire Slayer
Angel
Heroes
X Men
Lost
Doctor Who
Good Omens
Shared Actors and Plausible Character Constancy - I know your face.
Doctor Who & Life on Mars - Chameleon Arc + Tardis / Weeping Angels / etc.
Lost & Supernatural - Jacob & Lucifer
Fixed Fandom - You'll have to come to us.
Firefly
Lost
Planet of the Apes
Pitch Black / The Chronicles of Riddick
Example Crossovers
Watchmen and Ashes to Ashes - 1980's, here we come!
Dollhouse and Bones - FBI Agents and murders, oh my!
Human Target and La Femme Nikita - Undercover men and their L33T hackers face-off!
CSI and Terminator: The Sarah Connor Chronicles - Too many dead bodies lying around for this to end well.
Supernatural and ER (or House or even Lost) - We need a doctor, stat!
Angel and Lost - Physicists need love, too. Especially when they've been portal-and-time travelling.
Sample MultiFandom Crossover - 11 Fandoms
A string of unusual murders get Heroes' Matt Parkman involved (although The Company's already keeping tabs). Angel's Angel Investigations is chasing after rumours of disappearances and possession, with help from Buffy, The Vampire Slayer, and bump into Supernatural hunters doing pretty much the same. The multiple-homicide case is taken by Castle's homicide team (with help from the CSI team) until Bones's FBI Agent Booth gets a call from Dollhouse's ex-FBI Ballard over rogue-Doll Alpha suspsicions. Meanwhile Dollhouse's Senator (and Doll) Daniel Perrin requests Human Target assistance, but the anti-terrorist group of La Femme Nikita is also monitoring the situation, resulting in an epic hacker fight between Guerrero and Birkoff - and possibly Topher - which is spied on by Terminator: The Sarah Connor Chronicles John, keeping tabs on disturbing new technology which could lead to SkyNet. Topher looks on in horror as he puts the pieces together (and learns more about SkyNet than he ever wanted) and Michael outspies Chance.
Time-and Space Traveling - You can get there from here or now.
Watchmen: Dr Manhattan
Doctor Who: The Tardis
Terminator: The Sarah Connor Chronicles: The transporter
Heroes: Hiro ; Immortality
Supernatural: The Genie
Buffy the Vampire Slayer / Angel: Portals ; Unnamed "Normal Again" Demon ; Immortality
Life on Mars: hand-waving cars and bullets
Medicine - We'll take you there with flashing lights.
House
Grey's Anatomy
ER
Scrubs
Lost - Jack
Heroes - Peter, Emma
Criminal Activity - You have the right to remain silent.
Castle
CSI
Bones
Supernatural - Even if they're fake IDs.
Heroes - The FBI, The Company, Matt Parkman...
Angel - Angel Investigations
Human Target
Batman
La Femme Nikita
Life on Mars / Ashes to Ashes - Character's original timeline also viable.
X Men
Undercover - We're there even if you don't know it.
Human Target
Supernatural
Buffy, the Vampire Slayer
Angel
Heroes
Human Target
La Femme Nikita
Dollhouse - Not only is the house undercover, but the Dolls could be anyone.
Lost - Character backstories
X Men
Harry Potter
Good Omens
Watchmen - Within timeline constraints.
Batman
The Supernatural - We know weird.
Buffy, the Vampire Slayer
Angel
Heroes
X Men
Lost
Doctor Who
Good Omens
Shared Actors and Plausible Character Constancy - I know your face.
Doctor Who & Life on Mars - Chameleon Arc + Tardis / Weeping Angels / etc.
Lost & Supernatural - Jacob & Lucifer
Fixed Fandom - You'll have to come to us.
Firefly
Lost
Planet of the Apes
Pitch Black / The Chronicles of Riddick
Example Crossovers
Watchmen and Ashes to Ashes - 1980's, here we come!
Dollhouse and Bones - FBI Agents and murders, oh my!
Human Target and La Femme Nikita - Undercover men and their L33T hackers face-off!
CSI and Terminator: The Sarah Connor Chronicles - Too many dead bodies lying around for this to end well.
Supernatural and ER (or House or even Lost) - We need a doctor, stat!
Angel and Lost - Physicists need love, too. Especially when they've been portal-and-time travelling.
Sample MultiFandom Crossover - 11 Fandoms
A string of unusual murders get Heroes' Matt Parkman involved (although The Company's already keeping tabs). Angel's Angel Investigations is chasing after rumours of disappearances and possession, with help from Buffy, The Vampire Slayer, and bump into Supernatural hunters doing pretty much the same. The multiple-homicide case is taken by Castle's homicide team (with help from the CSI team) until Bones's FBI Agent Booth gets a call from Dollhouse's ex-FBI Ballard over rogue-Doll Alpha suspsicions. Meanwhile Dollhouse's Senator (and Doll) Daniel Perrin requests Human Target assistance, but the anti-terrorist group of La Femme Nikita is also monitoring the situation, resulting in an epic hacker fight between Guerrero and Birkoff - and possibly Topher - which is spied on by Terminator: The Sarah Connor Chronicles John, keeping tabs on disturbing new technology which could lead to SkyNet. Topher looks on in horror as he puts the pieces together (and learns more about SkyNet than he ever wanted) and Michael outspies Chance.