Here’s an idea.
Tywin Lannister dies at the Battle of the Greenfork.
A stray Stark arrow hits him in the neck. A freak hit.
The Lannister’s still win the battle.
I imagine the Lannister’s are shocked at this Pyrrhic victory. News would come in of Jaimie’s capture. But Tyrion would be officially heir.
The Lannister’s now have lost their head, but Tyrion and Kevan are still alive.
With Jaimie captured, and Tywin dead-the westerlands now has a succession problem. Tyrion would officially be heir and Tywin never disowned him officially, though he subtly insinuated to the lords of the west not to take Tyrion with respect.
I don’t think Kevan would usurp his nephew, but it’s a time of war. So putting power in the Imp’s hand is would make many of the westermen uneasy.
I do think Tyrion would likely assert his authority, and the westermen would not immediately oppose him.
So there are a few questions, if Tyrion is lord of the rock who goes to King’s Landing? Tyrion? Kevan? Lord Marbrand?
I’d think the Lannister host would still go to Harrenhall. Tyrion would probably not carry out the same chevauchee.
So...to make it interesting, Kevan would head to King’s Landing, with say 500 to 1,000 men.
Enough to bolster the garrison and put Cersei in her place.
While Tyrion held Harrenhal.
What would Tyrion do differently?
Probably more quills and ravens. He would send letters to the Ironborn, and also try to contact the Reach as soon as Renly died. The mountain clansmen would be used as Gregor was, for raiding and plundering the Riverlands. Tyrion would likely draw to Robb into a battle before the Walls of Harrenhall.
Robb didn’t take any such bait in canon. Would he here? I don’t think it’s guaranteed he wouldn’t. Given prejudices towards dwarfs and the Starks’ low opinion of Tyrion. “Crush the Imp and hang him by his toes on the walls”-Lord Umber
Though Kevan would more easily manipulated by Littlefinger and Varys. (Though Tyrion was too).
Ultimately, this all depends on if Kevan can hold Kong’s Landing. And if the Tyrell’s join team Lannister. If they don’t, then Stannis likely takes the city. Though Kevan will likely manage KL without some of Tyrion‘s faults. And might just hold King’s Landing even without the Tyrell’s. Sansa ends up married to Lancel. Though Stannis will likely retreat with more of his forces in tact. Assuming Oxcross still happens, Tyrion likely doesn’t give assurances to Lord Frey and Bolton. So Robb doesn’t die. At most Tyrion may get Lord Frey to defect and bar the Twins to Robb.
Beyond that I can’t really say.
Except the Tyrells either don’t sign on with Joffrey or they do later.
Stannis has more of his forces intact(assuming the siege is still unsuccessful-merely Kevan holds the walls and Stannis can not breach the gates-doing better than Tyrion).
So potentially Stannis doesn’t go to the Wall. At least at the same time. Leading to the wall falling to Mance raider.
Robb Stark fights through the Twins, in especially brutal and ferocious fighting. The Frey’s backed by Lannister promises and their own fury. Leading his tattered army, north. Hearing that his bastard brother was slain and the wall overrun.
Maybe Stannis sacks Claw Isle instead. (Say Davos dies at the Blackwater). And begins to hire eastern mercenaries, while also still holding Storm’s End. And many of the Stormlords remain loyal to Stannis.
I expect Tywin dying doesn’t change Robb’s immediate strategy too much.
Anyway early 300 AC-the war is a stalemate, with stark forces in the riverlands and north, the Lannister’s hold the capital, with the Tyrell’s joining after Stannis’s retreat. (Basically they pull a Tywin except for the violence). And Stannis holding the narrow sea isles, storm’s end, and several other castles in the Stormlands.
@The Immortal Watch Dog,
@Ganurath thoughts?