*assumes Freud glasses*
Clearly Aang flying upon Appa is the start of the dream and holds no deeper meaning, as with all things, we must start at the shallow end and work our way to what our subconscious holds.
The visions truly begin when Momo, now a giant rivaling the size of the Stay Puft Marshmallow Man, clearly indicates that Appa will be tossed to the side as the dominant animal in the show and that Momo will assume the role of the creature that carries the greater plot impact. His gripping of Appa means that he shall, in fact, be the end of Appa.
Aang's appearance before the Fire Castle is simple really. This will be the place of battle, the climactic end, and the place in which a large series of yellow donuts shall stream from a red sky. These donuts symbolize that Aang will be hungry though out the entire fight and this will be a great burden upon him.
Toph's blindness and disappearance into a black hole clearly means it is also the end of her. Her feet shall be smote with the fires of a thousand matches and she shall slip, falling down a laundry shute and remain incapacitated for the duration of the battle.
Sokka's shocked expression means he saw Iroh without pants.
Katara's presence in the middle of a flame circle signifies the weakness of her bending. In the dark confines of the fire palace during this event, she shall be seeing everything in shades. She will use whatever water sources are available and attempt to douse the fire nation's powers! But alas, it turns out that water, was in fact, OIL! She shall be her own doom.
Aang's frozen state and the giant Firelord are clearly opposites and signify the impossibilities of the world. Aang has reached a state of absolute zero in the presence of a giant, burning man which completely contradicts all the laws of chemistry. Thus, time and space will be warped and Aang's water shall suddenly melt, dunking him in a huge water pool!
The sudden appearance of Zuko and the comet clearly mean that the comet is related to Zuko. The real comet is not the giant mass hurtling through space, but it is Zuko himself! His burnt eyeball is the comet that shall bring doom upon the world! Aang must seize Zuko's face and cure it lest the world be destroyed!
Momo's appearance and then the sudden showing of the comet destroying the world signifies that if Aang is silent concerning Zuko's eyeball rock then the world shall explode, as silence to speak the truth signifies the lack of will to act!