http://www.fanfiction.net/s/5398503/52/Embers
Embers by Vathara, A very AU Airbender fic
Shorter chapter, some interesting material in this A/N.  I had most of this written out last night when it updated, but then had to go to bed without posting due to not being able to log in.
Notes written before having read 52:
A note on pacing:  
  The Embers alternate narrative starts with 'The Cave of 
Two Lovers' E2, and as of Chapter Fifty One (565k words) is covering cognates to 
'Sokka's Master' F4.  There has been strong foreshadowing for events covered in 'The Day 
of Black Sun' F10 and F11, so the story is unlikely to end before then.  Given the scale 
of the problems, and the implied desired methods and end state,  I am expecting the 
narrative to last to 'Sozin's Comet' (F18-21) or later.  I expect Embers to end up 
huge.
Further explanation:
Embers can be considered a cross over with the nastier grade of fairy tale.  The setting is one suitable for telling stories of supernatural horror in.
It is known in canon that there have been many Avatars, and that while there may have 
been a time with bending and no avatars, there was certainly an earlier time before 
humans had conventional bending.
In Embers, there have been hundreds of avatars, each with the potential to live 
half a millenium or more.  This means that a conservative estimate for the history of 
the Avatars is greater than our own recorded history.  I tend to go with an estimate of 
a hundred thousand years, and consider it middle of the road.  
In Embers, Humans settled the Avatar's world from elsewhere.  This time period may have 
been the one when the offense which motivates certain spirits occured.  (Certain spirits 
have been offended by humans, and think that wiping humans out will resolve this.)  This 
may also be when the Great Spirits were named by humans and had their natures redefined 
by the process.  Some time after coming to the world, the population started to seperate 
to align with the animals they learned bending from.  Then a plague occured, which seems 
odd, as these humans seem to have had at least comtempory technology, as well as what 
was either very advanced healing technology or magic.  Then the wars really started.
That the Avatar system in Embers was able to keep any of the four populations from 
wiping another for a hundred thousand years or so says a lot about its power to me.  
That the wiping out humanity plan has come along so far in so short a time against such 
speaks of the determination of its backers.  Frankly, as we as readers are just now 
getting into the meat of the whole mess of plans and counterplans, I expect it to take a 
good long while for the characters.
For those who are worried that nothing has happened beyond revelations of ever more 
complicated layers of plans, here is some stuff that has happened in chapters 45-51.
We learn more about Humans being alien to the Avatar's world.  We learn about 
the Volcano of Doom.  We meet Ozai's surviving Grandmother.  Ozai seems to think that 
Azula may be too stable and psychologically normal to serve as heir.  We finally meet 
Piandao.  And of course we also find out about the couple whom I now more strongly think 
are Aang's biological parents.
Reflections on a Dark Road by Chryssalid-Lathis, a crossover of Cap'n Chryssalid's 'Road 
to Cydonia' and Lathis' TTLB and 'Dark Titans' Teen Titans series.  
Ranma/X-com/Ranma/Teen Titans.