I stick by what I said of the comet. Oh, but you know what's going to get in the way and cause fights between everyone wanting something to do in the movie? Couples... or we could just eliminate them all together...
My list of ideas so far:
1. At the end of season three, (assuming this idea isn't actually used in the show) when Aang destroyed Sozin's comet he destroyed the "prison" of some kind of evil spirit.
2. This spirit, now released, seeks a way to return to the spirit world and take revenge upon all of those who imprisoned it. The only way this spirit can find to get there, is through Aang, whom he/she/it tries to capture.
3. I think we shouldn't hint at any couples at all, so as to avoid problems between all the different shippers helping in this.
4. Every major/semi-major character should make an appearance. (Ex: Major-Aang; semi-major-Jet)
5. I think it should feature a/some brand new enemy/enemies of our own creation.