Someone posted a Top Ten list of fan fiction he least wanted to see.  Well,
he only scratched the surface!  The creative universe is full of bad fanfic
concepts, if you know where to look.  For example:

RIDING KIKI - After settling in Chicago, Kiki is a victim of a vicious
"broom-jacking".  She hires Bean Bandit to get back her precious property,
and soon decides on her "witch speciality": shooting punks in the face with
hollow point ammunition!

Other "Elseworlds" sagas include:
Tenchi Mu-"YO!" - This alternative version takes place in Hollywood's
favorite economically disadvantaged community, South Central L.A..  Instead
of being sealed in a cave, Ryoko has been locked in the trunk of a junked
car for fifty years.  Other characters include Mimi Hoshi, a good cop who
saves the hero from a nasty beating at the hands of the Galaxy Police, and
in place of Kagato, Rapmaster Kozmik K, leader of the West Betelgeuse Crips,
and his ship "Sister" Soujah.

Oivey My Goddess! - The Goddesses land in a Jewish neighborhood in New York
City.

More bad crossovers:

Sailor Luna Varuga - Usagi wakes up to find a dinosaur attached to her butt.
She's mortified, as it just doesn't go with her costume.

Urotsukidojinshi - Demons attack the Comike and devour most of the guests.
The rest applaud, thinking it part of the entertainment.

Urusei Yawara - Watch out, Darling!  Lum's learned Judo!

Nadiaa of the Mysterious Valley of the Windy Seas - a combination of two
eco-happy anime, this one features tips on fly-fishing with Ohmu as bait.

City Insect Hunter - Ryo Saeba is magically shrunk to pixy size and meets up
with some willing women, only to wish he hadn't...

City Devil Hunter - Ryo is recruited by Yohko to deliver a magic bullet into
a devil's heart.  He does well as a Devil Hunter, but looks really dorky in
the dress.

All Purpose Fashionable Cat Girl Yawara - After his judo-champ daughter
suffers brain-death after an automobile accident, a scientist uses the brain
of a cat to revive her.  Now lovely and sweet Miss Yawara Inokuma is a
fashionable cat girl!  Meow!

Iczer 1/2 - When the Cthulu threaten Furinkan High School, 16-year-old
Nagisa Tendo splashes cold water on herself to transform into dimensional
warrior Iczer 1/2!

Kimagure Orange Roadbuster - After marriage, Kasuga and Madoka move to
Chicago and become mercenaries.  Madoka sounds cute saying "Eat lead, punk!"
in a thick Japanese accent.

Bubble Gundam Crisis - Mega-billionaire Quincy Aznable has Mega-Tokyo under
his grasp.  But fear not, here come the Newtype Knight Sabers, wielding
30-foot-tall "hardsuits" and driving motorcycles the size of a city block.
Of course, they do more damage then the bad guys, but hey, you can't make an
omelet blah blah blah...

Gundangaiorguss - Three popular mecha series mixed into one incomprehensible
mess.

And let's not forget these particularly malodorous cross-cultural
crossovers!

Rose Is Rose of Versailles
Lum-Bada - That Obnoxious Forbidden Dance
Bio Booster Armor MacGuyver
Space Ghost in the Shell
Lensmencrafters - "Saving the universe in about an hour!"

Man, I could go on forever!  Okay, I'll stop.  :-)