Cross posted-my second last post on the ASOIAF thread on SB.
Here's a WI
Indecisive Battle of the Trident
Rhaegar and Robert are both wounded but survive and both sides suffer around say 8,000-12,000 men killed each.
And so they withdraw and set up camp on opposite sides of the river.
Tywin already has seized KL-but there is no victor to hand the city to.
The way I see it, this can go a few ways, the two armies may clash again the next day or a few days later or both may withdraw and lick their wounds and summon more support.
This puts Tywin in an awkward position, he holds the city but hasn't declared for anyone(presumably). I am assuming he would have a contingency for an indecisive battle but he may not have. The only way logistically for Tywin to have reached KL was for him to have marched before the battle(during the weeks and days prior). And plan accordingly for whoever won.
Assuming no wildfire kaboom-Aerys will be in his former hand's custody, and so will Elia and the royal children. I only think Tywin would have them killed-if he was certain of Robert's victory, but if he marches to KL and enters the capital-forcibly or otherwise with reports coming in of no victor-he would not burn his bridges.
Either a second day of battle happens, or the war enters a longer protracted period-perhaps with both the rebels and royalists deciding to avoid another decisive battle or at least try to summon more support and dance around each other and provoke the other side into attacking first.
Politically speaking, Tywin has badly hurt his reputation both ways, his refusal to take a side and it being obvious he intends to play kingmaker and set terms to the victor cools both sides to him-thus neither Robert nor Rhaegar will be much inclined to give him what he wants. Rhaegar will be angry he took the capital and didn't march to his aid, and Robert will be angry Tywin obviously has no real loyalty to the rebel cause and just wants to cash in if he wins.
From this set up-you can have another battle either at the Trident or elsewhere to settle things decisively or have both sides in stalemate in the Riverlands. Tywin holding a restive capital and now having realized he has angered the victor whoever they are.
I don't see Tywin choosing a side at this juncture-as it will be seen as an insincere "Johnny come lately" act, but holding the capital for the victor and having their guaranteed ire regardless, also he would want to keep his options open and only side with the winner, not throw his already besmirched support behind a potential loser. Maybe both sides leave the Riverlands and try to seize the capital. Or choosing to fight-but that leaves their forces potentially weakened with a fresh Lannister army in control of KL. And thus able to hold it in the event of a siege or have even more leverage over the victor. That would definitely put commanders on both sides in a bit of a bind.
But thoughts?
TLDR: Tywin seizes the capital as canon but Trident is indecisive and both armies are intact, so they have to deal with Tywin now having set himself up as kingmaker holding the capital awaiting a winner.
If Rhaegar wins-he can punish Tywin for imprisoning his father and mother, if Robert wins-Tywin gets less to nothing, given he took the capital and only declared for Robert afterwards.
I could see Rhaegar moving towards the capital-to rescue his family, if he did not he looks weak to his supporters. Which means Robert can attack him from the rear.
You could have a lot of drama with both sides waiting for what choice Tywin might make-if he sides with the Targs, then the rebels have to prepare for a long siege with fresh troops defending the capital as well as Rhaegar's army being reinforced. If he sides with the rebels-Rhaegar is in between a rock and a hard place.
Given the fog of war-I don't find the above scenario too implausible. And I always found the Lannister's march to KL in RR to be logistically SOD breaking.
@Ganurath thoughts?