#he wants to die so bad so it means I outlive him mayhaps but it's fine thas why the ring it will be a good prenup

[shandykey]

sigh, time to get engaged I guess....

[joannalannister]

I was asked a question on Quora, but I fear I must ask for your help with it. How would the story be different if Tywin were alive? For ease, let's say Tyrion never climbs up that ladder and everything else is the same. Although this is painful, Tywin's death was perfect thematically.

[Casterly Rock] is where you should be. Tommen should accompany you, as your ward and squire. The Rock is where he’ll learn to be a Lannister, and I want him away from his mother. I mean to find a new husband for Cersei. […] once I convince Lord Tyrell that the match does not threaten Highgarden. 

“All he wants…” “…is to force me from the Kingsguard and send me back to Casterly Rock.”

“That need not be so terrible. He is sending me back to Casterly Rock as well. He wants me far away, so he’ll have a free hand with Tommen.”

“I am the regent,” she reminded him.

“You were. Tywin did not intend that you continue in that role. He told me of his plans to send you back to the Rock and find a new husband for you.”

I think if Tywin is still alive at the beginning of AFFC, then Tommen marries Margaery and is sent to Casterly Rock, with Jaime assigned to accompany his king. (Tywin is still hoping to get Jaime out of the Kingsguard, and getting him in Casterly Rock is one step closer to that.) 

Cersei is sent to Casterly Rock for a time, until Tywin can find a husband for her:

“You will marry and you will breed. Every child you birth makes Stannis more a liar.” […] "Mace Tyrell, Paxter Redwyne, and Doran Martell are wed to younger women likely to outlive them.“

Who that husband is, I don’t know, because I don’t have all of the information Tywin has on eligible bachelors in Westeros, and I don’t like speculating in the dark. Probably someone from the Reach or Westerlands. 

The Lannisters still have to broker a peace in the Riverlands, take Dragonstone and Storm’s End, fend off the Ironborn in the Reach, etc. However, I don’t think the Faith is armed, and with Cersei sent away, there is no walk of atonement. The Lannister-Tyrell alliance is under much less strain than it is with Cersei in charge. 

Prince Doran remains very cautious with Tywin around, but he still works for Tywin’s downfall.

“All Dorne will howl,” said Doran Martell in a tired voice. “I only pray Lord Tywin hears them in King’s Landing, so he might know what a loyal friend he has in Sunspear.”

I have worked at the downfall of Tywin Lannister since the day they told me of Elia and her children. It was my hope to strip him of all that he held most dear before I killed him, but it would seem his dwarf son has robbed me of that pleasure.

Lord Tywin had been a dangerous foe; without him, Dorne’s enemies would be much weaker. 

There is soon to be a food crisis that Tywin will have to figure out a way to deal with, since he burned all the food in the Riverlands:

Snow in the riverlands. If it was snowing here, it could well be snowing on Lannisport as well, and on King’s Landing. Winter is marching south, and half our granaries are empty. Any crops still in the fields were doomed. There would be no more plantings, no more hopes of one last harvest. He found himself wondering what his father would do to feed the realm, before he remembered that Tywin Lannister was dead.

In White Harbor, I think Wyman Manderly bends the knee, but secretly continues his suicide mission for vengeance:

Lord Tywin Lannister wrote me himself to say that he had Wylis. If I would have him freed unharmed, he told me, I must repent my treason, yield my city, declare my loyalty to the boy king on the Iron Throne … and bend my knee to Roose Bolton, his Warden of the North. Should I refuse, Wylis would die a traitor’s death, White Harbor would be stormed and sacked, and my people would suffer the same fate as the Reynes of Castamere.“

I am fat, and many think that makes me weak and foolish. Mayhaps Tywin Lannister was one such. I sent him back a raven to say that I would bend my knee and open my gates after my son was returned, but not before. There the matter stood when Tywin died. 

“It was Tywin Lannister who returned Wylis to me, safe and whole, as he had promised. A man of his word, Lord Tywin, Seven save his soul.” Lord Wyman popped the meat into his mouth, chewed it noisily

In the North, Roose Bolton is warden, Ramsay marries Jeyne, all that bad stuff still happens. I think Roose at least gives the appearance of loyalty to the Iron Throne. 

At the Wall, the Night’s Watch won’t recognize Stannis because of fear of Tywin’s retaliation, so Stannis will have a very difficult time:

“Lord Stannis helped us when we needed help,” Marsh said doggedly, “but he is still a rebel, and his cause is doomed. As doomed as we’ll be if the Iron Throne marks us down as traitors. We must be certain that we do not choose the losing side.”

“King Robert was well loved, and most men still accept that Tommen is his son. The more they see of Lord Stannis the less they love him, and fewer still are fond of Lady Melisandre with her fires and this grim red god of hers. They complain.“

In Essos, I’m not sure if Aegon would be able to sail west yet without the dragons, if Tywin were still alive. I think someone might convince him that Tyrion had a bad plan, and that going head to head with Tywin without dragons is bad. 

"I have had enough of Illyrio’s plans. Robert Baratheon won the Iron Throne without the benefit of dragons. We can do the same. And if I am wrong and the realm does not rise for us, we can always retreat back across the narrow sea, as Bittersteel once did, and others after him.”

Strickland shook his head stubbornly. “The risk—”

“—is not what it was, now that Tywin Lannister is dead. The Seven Kingdoms will never be more ripe for conquest. Another boy king sits the Iron Throne, this one even younger than the last, and rebels are thick upon the ground as autumn leaves.”

Also, I’m not totally sure that Tyrion would try this divide and conquer approach with Tywin still alive, because I think he might want a very large invading force before facing his father across the battlefield. 

Assuming Tyrion’s alive, like, idk, Tywin might hire a Faceless Man or s/t to kill Tyrion.

The Iron Bank is still a problem, so it might get to a point where Tywin needs to use Lannister gold, as he did under Aerys. 

And all of this assumes that Varys doesn’t kill Tywin in the near future to destabilize the Lannister-Tyrell regime to make way for Aegon. The Realm might ostensibly be at peace under Tywin’s continued regime, but it’s basically a powder keg and Tywin is Franz Ferdinand. I don’t know why Varys wouldn’t take advantage of that. 

Also, this is a slightly different form of a Tyrion Doesn’t Kill Tywin AU by @nobodysuspectsthebutterfly, where Oberyn wins and what happens after, which might be useful to you. 

I’m not very good with AUs, so other people might have better ideas.