SPOILERS ahead from the Game of Thrones series finale, and also GRRM's books.
On TV, a Great Council of the great houses of Westeros gathered to pick the new king. (And they went with Bran, which is whole other can of WTF.) Turns out, Cat actually suggested the Great Council ending to Renly back in the second book, right before Melisandre's shadow demon killed Renly and stole attention away from her idea. The TV show didn't quite cover the scene the same way. Here's how it played out in the book:
Catelyn's Great Council Pitch In The Book
In a Catelyn chapter of A Clash of Kings, Cat went to "King" Renly's camp to make her pitch for kings Robb, Stannis, and Renly to come together to take down King Joffrey through exposing Cersei Lannister's incest with Jaime. Cat told Renly the Lannisters tried to kill her son Bran, and she had asked herself a thousand times why. But Stannis gave her the answer when he said Cersei's children were the result of incest with her brother. That's when she got to the paragraph that suggested the kind of Great Council we saw in the TV finale. Here's what she told Renly:
'Robb will set aside his crown if you and your brother will do the same,' she said, hoping it was true. She would make it true if she must, Robb would listen to her, even if his lords would not. 'Let the three of you call for a Great Council, such as the realm has not seen for a hundred years. We will send to Winterfell, so Bran may tell his tale and all men may now the Lannisters for the true usurpers. Let the assembled lords of the Seven Kingdoms choose who shall rule them.'
Renly laughed in response:
'Tell me, my lady, do direwolves vote on who should lead the pack?'
So he laughed it off, ala the lords and ladies laughing off Sam's idea of a fully democratic vote. Brienne was dressing Renly for battle during that speech, so she heard the pitch for a Great Council. Renly is now dead, in the books as well as TV, but Brienne is alive, and if she gets to the end of the books as well, perhaps she'll recall Lady Stark's talk of a Great Council and that's how we get to that idea?
How The Cat, Renly, Brienne Scene Played On TV
Things played out a bit differently on TV.
In Game of Thrones Season 2, Episode 5, "The Ghost of Harrenhal" started with Catelyn saying Robb had no desire to sit on the Iron Throne. Renly said Robb could continue calling himself King in the North, and the Starks will have dominion over all lands north of Moat Cailin. But Robb would have to swear an oath of fealty to Renly. The wording would be the same Ned swore to Robert 18 years before. Their friendship held the realm together.
Renly said the Starks and Baratheons are natural allies and together they could end this war in a fortnite. Then the shadow arrived.
I know many book-reading Game of Thrones fans are disappointed that the show skipped the Lady Stoneheart storyline, but the show also left out Cat's Great Council pitch, which turned out to be fairly prophetic, as the series turned out. (Book readers send their regards, Lady Stark.)
How The Book Series Will End
George R.R. Martin isn't done with his series, he still has two more books left to finish. It'll be interesting to see what he does with the ending, now that TV viewers have seen the HBO versions' chosen finale. Will Brienne bring up the Great Council idea, remembering Lady Stark's pitch, or will someone else have a similar thought toward the end, if GRRM's final book, A Dream of Spring, goes for the same angle?
Isaac Hempstead-Wright had this to say about the book-and-TV ending for Bran Stark, during an interview for HBO’s Making Game of Thrones:
[Showrunners] David [Benioff] and Dan [Weiss] told me there were two things [author] George R.R. Martin had planned for Bran, and that was the Hodor revelation, and that he would be king. So that’s pretty special to be directly involved in something that is part of George’s vision. It was a really nice way to wrap it up.
So Bran is also going to end up king in the books, as far as Isaac Hempstead-Wright was told. It'll be interesting to see if George R.R. Martin goes about it via the Great Council route as well. I imagine he'd be able to build up to it more, so it won't feel like it came out of nowhere like on the TV show.
Our watch has ended for Game of Thrones, and HBO has made it clear there will be no direct sequel spinoffs like following Arya west of Westeros. However, maybe we will see each other again in 10 years time, if that one idea pans out. And we know HBO has at least one prequel series hitting the ground fairly soon.