The crossover is a staple of science fiction fandom: it's the only way to answer age-old questions like, "The Enterprise or the Millennium Falcon?" There have even been canon crossovers, some of which have gone well and some of which...didn't.
By their very nature, the options for crossovers and fusions are endless. Here are ones that never made it to screen or page, but did make fantastic art.
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Top image: Downtime in Spacetime by Either-Art at deviantART
Ten and Wheatley by radiohamlet at deviantART
HP-Naruto crossover by Phii-chan at deviantART
Mis-firing Portkey by tripperfunster at deviantART
WWACC Tron Pilgrim by dyemooch at deviantART
The Doctor and Marty McFly by Obsidian at Commissioned Comic
Predators by JoannaJohnen at deviantART
The Host 2 by Leah Rosenthal
SCI-FI CROSSOVER by crisdelara04 at deviantART
Simply walk into Mordor by ferret42 at deviantART
Muppets Kick Ass by StrangelyDrawn at ArtBreak
The Prince and the Lord by JaviDLuffy at DevianART
Ahsoka and the Beast by clc1997 at deviantART
hulk vs hellboy by gabahadatta at deviantART
One of the Star Wars Movie Posters by Matthew Ranzetta
Batman VS. Darth Vader by Dave Dorman at Loyal K.N.G.
Star Trek vs Dr. Who by Summerset on deviantART
Part of the Mural by Jim Mahfood at http://www.ign.com/IGN's office, via Comics Alliance
Collab of Badassery by Daniel Araya and Enrique Rivera via Venture Bros. Blog
Black Shirt Boogie by rahzzah featuring Brock and Wolverine via Venture Bros. Blog
Darth Vader vs. Aliens by David Hillman, inks by Mark McKenna and colours by Tom Chu via How to Carve Roast Unicorn
Rodney McKay as The Doctor by WolfenMoondaughter at deviantART
batman sesame street by Matthew Benkner via you love meg
Spacetime's Finest by Dean Trippe at etsy
Yellow Lantern Zim by Arqueart at deviantART
Poker Game by Stardamsel at shirt.woot
Oh No: Superman UK by Aviv
USS Millennium Falcoln by Phil Noto
SPN - Hellboy crossover by JoannaJohnen ay deviantART
The Dagobah Connection by Peter de Seve via Loyal K.N.G.
Law & Order: UFP's Detective Odo by Jake
The Doctor meets the Doctor. by Furrysketch at deviantART