I'll tell you, I would kept almost all of the episodes so the movie would be at least two or three hours long.
The casting of of course asians. I would even cast Jackie Chan as Uncle Iroh, Chow Yun-Fat as Zhao, and Jet Li as Ozai. Also I would as a prequel arc that's about Aang. If the studio asked me to do it in 3D, I'll say yes. But only with legit 3D cameras. No conversion from real d. Plus I would want to put subtitles in for Appa and Momo when they have a Conversation.