It should be just awful

My theories ended with Tywin having used all the gold in Casterly Rock to hire a bunch of faceless men(Littlefinger included) to seduce every Stark, and get them killed without any evidence of foul play.

Because he somehow got a hold of Maggy The Frog after Cersei came back without her highborn friend, a daughter of one of Lannisters' bannermen, who ended up being found dead in a well(cause Cersei pushed her out of jealousy and destined to happen by prophecy), and he most likely learned it all through torture of Maggy the Frog, reminiscent of the torture of Gollum, which secured himself a thousand years worth of prophecies, in order to secure his dynasty which would last a thousand years.

He shared these prophecies gradually with Jaime by forcing him to learn how to read, so that he could read secret notes about the future, burn them and never speak of them, so that they could secure the Lannister legacy, a dynasty that would last for a thousand years(also contributing to his endless arrogance). Basically by having Jaime impregnate Brienne before running away to die with Cersei and her fourth unborn child, a bastard would go through a similar adventure like Jon Snow, but be basically unstoppable, honorable, tall and handsome which would be a preferrable heir to the Iron Throne after years of a mad Targaryen Queen who enjoys burning anyone who objects.

That would explain everything. That would actually make the Lannisters look like a threat equal in power to the Three Eyed Raven. Bran may be able to see past and future events, but not everything, nor was he omnipresent. While the Lannisters would focus on securing their legacy, which would live on beyond the lives of the sheep and truly become a glorious dynasty.

This is not just wishful thinking. I found all kinds of stuff that would inevitably lead to this if only the show went on. Sam was given a book on the entire history of the Lannisters at the Citadel. Sam stared at Jaime at the battle of Winterfell because of it, because he had read and he knew that Jaime would survive, just like Bran who asked Jaime how he knew "there will be an after?".

It would even explain Cersei's complete arrogance after becoming queen, her reactions to the wall coming down would be justified if she as Queen inherited her father's secret prophecies. She would truly understand that "knowledge is power".

Littlefinger didn't need to come back. He fulfilled his purpose. He was supposed to die as the scapegoat. The perpetrator of all of Tywin's plots. No one would know it was actually Bronn's valyrian steel dagger all along or that he started the fire at Winterfell and armed the cutthroat with his own dagger. Just like Talisa who made up her last name on the spot in front of Catelyn Stark, would lead to the Red Wedding. Just like Ygritte, a male wildling with a male voice until Jon Snow flipped her around(watch it), would lead to Jon watching her die. Just like Shae would lead to Tyrion's sentence.

He was going to fuck them. And he did. Tywin that is. Littlefinger merely represented the ugly side of Tywin's will.

Tyrion was on to it from the start, to the point where nobody have even noticed that his drinking game is actually an interrogation technique. Or that Sansa's ship watching game was an equivalent without the need for alcohol.

But I'm not even going to attempt to describe it. The show is over. George R R Martin better take notice of how the ending is not even close to what the show could be. I hope he writes what I believe he was originally planning to do, or something even better.