So the more I read about it, and the more I think about it, M. Night's still trying to cram too much stuff into each of the movies. The initial reviews of things feeling rushed seem confirmed on this.
One interview with M. Night confirms that Haru's village will appear in the first movie - but without the giant metal prison. Suki has also been confirmed, but there's no word yet on Jet and Teo. He's moving Bumi to the 2nd movie, and JeongJeong to the 3rd; which just means more stuff to cram into those movies.
I have to wonder why he's leaving Haru's village in, since that would probably be one of the first things I would cut.
I've been thinking, that had I the time, I would try and make an edit of the season down to an hour and a half movie - two hours max. But there would be a ton of things that would get left out.
If I had to do it, the first three episodes of the series would remain mostly intact, but condensed for time. But I'd move some of the events that take place at Crescent Isle to the Southern Air Temple, having Aang learn about Sozin's comet from Roku there. Some of the flashback scenes from The Storm would be interspersed during the Temple and Agni-kai scenes. Zhao would capture Aang from the temple, forcing Zuko to don the Blue Spirit persona. Then skip immediately to the Northern Water Tribe for a condensed version of the last three eps.
No Suki, Bumi, Haru, Hei-bai, JeongJeong, Jet, Bato, Jun, or Teo... But that's about the only way I could see getting the essence of the show with enough character building for the main characters without it feeling too rushed.
The Second movie presents a bit more problems as far as just making a condensed version of the 2nd season, especially given that I would have cut Jet and Suki out of the first movie.
M. Night said he's going to leave the Lost Appa part of the story in, but I'd honestly take it out. After leaving the North Pole, Aang would meet Toph pretty much right away, but Azula and the angels would crash Earth Rumble, leaving the Gaang to flee into the desert. After finding out about the day of Black Sun, they'd go immediately to BaSingSe (not having Appa stolen means they can just fly directly there) to get the Earth King to help with the invasion. I'd probably have Azula already in the city by the time the Gaang, and Zuko, arrive. The events of BaSingSe would be pretty condensed, stripping out "The Drill," "Lake Laogai," and most of "City of Walls and Secrets" and "The Earth King." Leaving mostly just the last two episodes. Zuko's journey through the earth kingdom would be pretty compressed, but the flash backs from "Zuko Alone" are pretty important, so they'd keep.
The Third movie is probably the easiest to condense down without too many problems. I'd take out the first half of the season almost entirely. Aang would wake up with the Gaang already preparing for Day of Black Sun, which would pretty much open the movie. Zuko would betray his father and join the Gaang, teach Aang Firebending, and then pretty much jump into the Finale.