Fanfictions!
First off let me say that as with any fanfiction I don't own
any characters or situations, but those I create myself.
Some consider Fanfictions worthless and wastes of time.  
After all you can't make money off of them right?
I beg to differ.  I have found them to be a useful tool to
practice with character and background development
using a template already established by the Movie/TV
show/Book.  
Another advantage of a Fanfiction is that the audience
for them tends to be built in, therefore the feedback rate
is quite a bit greater than that of an original fiction.
How does that help you ask?  Well the amount of
feedback equates to greater exposure, gives tips on your
writing in a mass audience, and allows for growth.  It is
an invaluable tool!
So I offer my "what ifs," inserted characters, and
exploration of new events for your perusal!  All of them
against familiar backdrops!
Underworld
Predator
Dark Angel
Gargoyles