A Different Weasel Makes a Difference...or Old Walder Dies Early
Here's another fun story from the alternate-history forum. The world of A Song of Ice and Fire is so complex and so many roads not taken that playing the alternate-universe game can go in some pretty cool directions.
So I present you with A Different Weasel Makes a Difference. It diverges from canon just before the War of the Five Kings breaks out, during the events of A Game of Thrones. Lord Walder Frey dies soon after wedding his eighth wife, making Stevron Frey the new Lord of the Crossing. Soon afterward, Jon Arryn is poisoned and Gregor Clegane unleashes his reign of terror in the Riverlands, kicking off what will become the War of the Five Kings. Or, as he calls it in the story, the War of the Eight Kings...
There are all sorts of interesting consequences. The change in the lordship of the Twins sets the butterfly wings a-flapping, spawning more changes from the canon timeline as the war goes on.
*A Mallister ship is lost to "unknown causes" in the Sunset Sea, alerting the Starks and their allies that the Greyjoys might be moving again. Consequently, Catelyn Stark is sent north as regent after Robb is hailed as king and the Starks make more preparations for possible attack from the sea, including reinforcing Moat Cailin.
*When King Renly Baratheon is killed by Stannis's shadow, his lover Ser Loras Tyrell impulsively leads a cavalry charge against King Stannis Baratheon's encampment near Storm's End. Sufficient to say, impulses have their drawbacks...
*Stannis's sorceress Melisandre unleashes her magic on an army-level scale, fueled by mass human sacrifices. It is terrible and it is awesome.
*The battle for King's Landing is a multi-chapter morass of mayhem called the Battle of the Four Armies. A nice little call-back to The Hobbit there... :)
*We have the morbid irony of the armies of Robb Stark--a secessionist from the Targaryen-created Kingdom of Westeros--enforcing Maegor the Cruel's laws against the arming of the Faith. It'd be like Polish rebels enforcing the laws of Russia, Prussia, or Austria-Hungary on their political enemies.
*The long-oppressed thralls of the Iron Islands get their moment of glory.
*Asha Greyjoy does something radical and totally awesome that changes the balance of power in Westeros forever.
*The Vale isn't nearly as peaceful as canon. Hint: Somebody definitely earns the title "the Mad Widow."
Although the fanfiction.net version of the story is behind that on AH.com, there's still a whole lot of material present and it's getting updated regularly. So check it out!