LotR: RotK (I love acronyms!) to me gets marginal points as a "non sequel" but they did write and shoot BttF 2&3 together, Matrix 2&3 togther, PotC 2&3 together, and to some degree there were notes and plans for RotJ when ESB was in production. The Prequel Trilogy was a planned trilogy and so was Harry Potter (Septology?)... so I'm not sure that there's "Return of the King" and "Everything else" as far as sequels not being sequels or sequels that are better than sequels... but I digress.

Whether it is a 2nd sequel or the 3rd non-macro act... it still feels very wrong to introduce the ghost army at that point in the story. I know the books are also like that, but they had 50 years to figure out how to massage that problem in the movie. I wish the prologue would have drawn attention to that kingdom deserting the war of the end of the 2nd age. At least then it would have been referenced, hinted, or foreshadowed sometime before the 10th hour of a 12 hour epic.