"Unity of plot does not, as some persons think, consist in the unity of the hero. For infinitely various are the incidents in one man's life which cannot be reduced to unity; and so, too, there are many actions of one man out of which we cannot make one action. Hence the error, as it appears, of all poets who have composed a Heracleid, a Theseid, or other poems of the kind. They imagine that as Heracles was one man, the story of Heracles must also be a unity. But Homer, as in all else he is of surpassing merit, here too- whether from art or natural genius- seems to have happily discerned the truth. In composing the Odyssey he did not include all the adventures of Odysseus- such as his wound on Parnassus, or his feigned madness at the mustering of the host- incidents between which there was no necessary or probable connection: but he made the Odyssey, and likewise the Iliad, to center round an action that in our sense of the word is one."
For example, the most reasonable view of Agamemnon's negative remarks concerning Kalchas at Iliad, Book 1, lines 123-129 (H&P's line numbers, p. 380) is that Agamemnon is alluding to Kalchas' prophecy concerning Iphigenia nine years before (for this, see H&P's paragraph, "The Events at Aulis", p. 362).
Another point which H&P refer to as in some sense not being mentioned is the imminent death of Achilles. Actually, this is a dominant theme, especially in the latter part of the poem, as for example in Book 21, lines 1-3 (H & P's numbering, p. 424) and in Book 22 (H&P, lines 333-338). In fact, as early as Book 1, Thetis (speaking to Zeus), implies the same point (H&P, p. 390, lines 578-579).
Also – although one needs to remember that the Iliad ends with the funeral of Hektor, before the actual capture of Troy –, the doom of Ilion (Troy) is strongly prefigured in passages such as Hector's observations in Book 6 concerning Andromache's eventual fate (H&P , p. 398, lines 74-94). Particularly notable is the beginning of this passage, lines 74-76.