Obviously, everything here is the property of George R.R. Martin. I own nothing except any original characters.
The First Kingsmoot of Westeros
This is an alternate history of George R.R. Martin's fantasy series A Song of Ice and Fire. It's based off a single line in A Clash of Kings (Book 2) where Catelyn begs Renly to not fight Stannis, to ally with him and Robb and after the War, they may decide amongst themselves who shall be King.
Renly's response is, "Do wolves decide which among the pack will rule? No."
But say Catelyn's plea hadn't been so desperate. Say Catelyn had been a little more persuasive and brought a more solid plan to both Renly and Stannis. Admittedly, it's a stretch with our King in the Narrow Sea, but I've caught election fever and am trying to channel that energy into a Westerosi TL, so here goes.
The POD is mid-298 A.L. Catelyn Stark joins Renly's forces as they meet with Stannis' in the Stormlands. When Stannis pulls out Lightbringer, Lady Stark jumps in front and eloquently brings together the Baratheon brothers.
When the Red Woman declares that Stannis need not consult with Renly and Robb, Catelyn retorts, "Then what does he have to fear of a mere council?" She convinces the Baratheons that the Lannisters are the true enemy and begins an Allied war council.
The Baratheon-Redwyne fleet journeys around Dorne and into Blackwater Bay to blockade King's Landing. Renly marches up the Rose Road while Robb brings the Stark host across the River Road. On a field south of the God's Eye, Tywin Lannister makes an epic stand and gloriously falls to the Allied forces of Kings Renly and Robb.
The Stark-Tully-Baratheon-Tyrell combined force then marches on King's Landing. Sandwiched between the Allied army to the west and the fleet to the east, Tyrion Lannister is no fool. He entreats with Lady Stark on the surrender of King's Landing. They come to an agreement:
The city shall not be harmed.
All Lannister bannermen who lay down arms will be free to leave the city and go home, all others will swear fealty to the Small Council.
The false King Joffrey Baratheon and Queen Cersei Lannister will be put to trial by the Small Council.
Tyrion Lannister, as defender of the city, will be granted a place on the Small Council of the Iron Throne, until a new King is chosen.
The agreement is made and the Allied forces enter King's Landing, ready to distribute food and security. Joffrey and Cersei are thrown in a pair of Black Cells as the Kings debate on control of the city. Robb is all to eager to return north with his army and seize Winterfell from the Greyjoys. Catelyn Stark once again comes to the rescue with a compromise: King Robb and King Stannis will bring the Allied armies north (Robb through the Neck, Stannis to White Harbor to flank Moat Cailin).
As Robb and Stannis forge a strange warrior friendship as they defeat Ironborn, Bolton, and wildling, their advisors are hard at work in King's Landing with Lord Renly and Tyrion Lannister on the Small Council.
Lady Stark, Lord Renly, Tyrion, also deal with Mace Tyrell and Alester Florent. Most of the Lannister bannermen have remained in the city to keep the peace and distribute food. Ravens fly almost endlessly from King's Landing to Oldtown as Maesters are consulted on the laws of the Seven Kingdoms of old and the laws of Aegon's Westeros.
Finally, the North is free of ironmen, Mance Rayder has been captured, and Ramsay Snow has been hanged. King Robb and King Stannis gather their lords to return to King's Landing and begin the First Kingsmoot of Westeros. What they leave behind is a land burned, sacked, and desolate. The walls of Winterfell black, the gates of Castle Black demolished, a bastard as Lord Commander of the Night's Watch.
So this is the situation at the start of our TL
King Robb Stark leaves the North victorious. Stark soldiers defend the Wall alongside the Night's Watch, now led by the 998th Lord Commander Jon Snow.
King Stannis Baratheon also leave victorious, half his fleet stationed in Skagos, the other half ferrying him to King's Landing.
Ser Brynden Tully waits in Seagard with an allied Tully-Tyrell force preparing for the invasion of the Iron Islands.
Lord Renly, preparing to sit the Iron Throne pulls strings to turn the Kingsmoot Council in his favor.
Joffrey Baratheon and Cersei Lannister have been executed.
Sansa and Arya Stark are long missing.
Theon Greyjoy is assumed dead.
Brandon Stark has gone north of the Wall, Rickon Stark is missing.
In the East, things have gone as IOTL. Daenerys Targaryen is about to seize Meereen, or has seized it already.
Shortly before we begin, Oberyn Martell and his Dornish host of 300 have entered King's Landing.
The pieces are set. The next round of the game begins.