I agree with everyone who says that at the end of the third Book, Aang will either not defeat the Fire Lord or simply not go against him. Frankly, he is not in a position to be going anywhere near the Fire Lord's palace. I am going on the end of the second season, and even though he has ninteen more episodes to make something happen (which is a lot of time), with the Earth Kingdom conquered, the Air Nomads killed, and the Water Tribes barely able to fend off one attack (and that was with Aang there) the possibility of Aang attacking the palace with only himself, Katara, Sokka, Toph, and whoever they find to be Aang's firebending teacher is laughable. Even though Books 1 and 2 were about Aang mastering water- and earthbending, Aang still hasn't done that. If you remember, in The Library, Aang says that he practices with Toph and Katara every day, meaning that he still doesn't have waterbending down, even though he supposedly did at the end of the first season. He still probably doesn't have earthbending mastered, even though it seems as though he does. The idea of a fourth Book is more plausible because it gives him more time to prepare, matser the elements, get Azula and the Dai Li out of Ba Sing Se and put the Earth King back on the throne, and help bring the Water Tribes together. It would also give him time to find more Air Nomads, if there are any which there probably are otherwise that would mean no more Avatar after another cycle.
Sorry if this is kinda long, but I had a lot on my mind...