One of the things I think they NEED to do though (assuming that ASOS is 2 full seasons, or close to it), is save the red wedding for season 4. The last third of ASOS, starting with the red wedding, are very much one big rolling conclusion to many of the character arch's and plot lines over the first three books. I feel that all of those events should take place in season 4. That is also safer for keeping viewership, as it will happen in the middle of the season and there is Joffrey's and Tywin's deaths, and Sansa's escape from Kings landing coming up to ease the pain of the Red Wedding. Then with AFFC and ADWD happening chronologically in the show, we should get Davos at White Harbor sometime in the next season to remind us that the north remembers, which for me was enough to get my excitement way up there for the rest of the series.
It will be very interesting in general to see how they translate A Storm of Swords into the TV series. There are just so many BIG events that happen in that book, and, like you said, a lot happens in the last third or so.
Off the top of my head,the last third of A Storm of Swords , from about the Red Wedding on covers:
- The Red Wedding and everything surrounding that (presumably that means they have to introduce the Tully's sooner or later)
- Joffrey and Margaery getting married
- Joffrey getting iced
- Sansa escaping and then making her way up to the Eyrie leading to Lysa getting iced
- The Wildling Battle at the Wall
- Bran and company meeting Sam and going beyond the Wall with Coldhands
- Dany setting up shop in Mereen
- Jaime coming back to King's Landing
- Tyrion on trial, the Red Viper vs. The Mountain
- Stannis showing up to save the day at the Wall
- Arya and Sandor going around, with Arya leaving eventually
- Tyrion icing Tywin and Shae
- Jon becoming Commander of the Night's Watch
- Introducing Lady Stoneheart
That's a lot of stuff to cover after the Red Wedding. The events at the Wall alone will presumably require a ton of work. The problem with the show would likely be the slower pace of things after ASoS, once they get back to more Dorne and more new characters. Compared to the first 3 books, AFfC and ADwD feel a lot like setup where not a ton of stuff happens.