ATF AU in Magnificent Seven fandom. This is an AU that is built around the members of the Magnificent Seven being ATF agents rather than Wild West Cowboys. It is an AU that caught on in the fandom, lots of people wanted to write ATF AU stories, to the point where some uninitiated readers thought that the ATF AU was the actual world the Magnificent Seven TV show was set in. Not the first time fanfic has overtaken the original show for some fans, but it is certainly unusual for an AU to do that.
There are some AU's where I wonder why someone wanted to create them, these are the ones like putting Harry Potter and all his pals into a universe without magic - hmm, yeah, kinda the raison d'etre for Harry Potter for me is that there's magic and wizards and witches and fantastical creatures - stick him into a normal high school and well, you have Adrian Mole ;).
Still, some people like those kind of stories, so all power to them. HP isn't the only fandom to do High School AU's either, in fact, it is only a matter of time before any fandom will have a High School AU, and I mean ANY, including those already set in High School! It's a standard trope and a favourite of fanfic writers, probably because a lot of them are still in High School.
Excluding those fandoms that are already set in High School, the standard method of creating a High School AU is to transport all the characters from your favourite fandom, both in age and situation, into school (mostly as pupils, but some may become staff). A bit like Disney is doing with Monsters' Univeristy, actually :D. Just like geologists examining their xenoliths to see, a/ how they got there, and b/ what that means to the xenolith, fanfic writers want to examine the effect of teenage hormones and school problems on their favourite characters.
The Mirrorverse in Star Trek began in original Trek and has popped up in different Trek incarnations and is also a popular fanfic universe for writers as well. Kirk/Picard/Kira being bad can be fun to read and write and watch ;P. DS9 had particular fun with the Mirrorverse. And, of course, the latest two Star Trek movies, although a reboot, could be thought of as an AU, since in the original Trek universe, Vulcan remains in one piece.
When I write fanfic, I don't tend to go in for AU's, because, for me, the reason I like writing fanfic is because I find something inspiring about the universe it's set in, e.g. magic etc with HP. Usually, if I have an AU idea for a fanfic, it will generally morph into original fiction, because I find the characters don't fit. This happened with a YA story from bandom, fanfic about certain rock bands, which was originally based on the premise that one member of a particular band was a werewolf. However, I never got very far with it as a fanfic and it became an original story, Samling-Born, which I developed as part of the Wittegen Press Giveaway Games.
So, all in all, AU's can be fun, lots of folks enjoy changing the colour of their sand pit and trying their favourite characters out in it. However, it's not my favourite type of fanfic. My xenolithic fanfics tend to become absorbed by the original-fiction magma around them until just the magma is left.
What do you think of AU's?
P.S. Check out other folk doing the A to Z April Challenge.
And if you want to see my other posts:
Sophie's Thoughts & Fumbles A to Z Challenge Posts