In reference to the Aeneid I believe it is absolutely possible for later works to be just as substantial as earlier works. The Aeneid is not a mere plagiarism of Homer's Iliad and Odyssey. It's true that Aeneas' wanderings in the beginning of the poem are reminiscent of the same perils that Odysseus faced on his journey home to Ithaca, such as Aeneas' encounter with the Harpies (3:245) and the Cyclopes (3:760). Aeneas then struggles to safely colonize the Trojans in Italy upon his arrival which mirrors Odysseus' rocky homecoming and slaughter of Penelope's suitors. The latter half of the Aeneid resembles the Iliad as war, spurred on by Juno, breaks out between the Trojans and the Latins in book VII and Turnus summons his warriors to defend their home (7:562) as Paris did at Troy. .
At first all of these similarities may appear to be a poorly veiled appropriation of Homer's epics, but that is far from the actual purpose of Virgil's epic. Virgil seeks to reveal the true reason for the Trojan War and Aeneas' journey to Italy to found the predecessor to Rome. He explains through the Aeneid that the underlying purpose for the Trojan War and the life of Aeneas has finally been fulfilled with the reign of Augustus in which the emperor managed to reunite the Roman Empire and bring peace to Italy after a period of constant civil war. This is evidenced in book VI of the Aeneid, which superficially seems like a complete imitation of book XI of the Odyssey. However, as Anchises describes "the glory that awaits the Trojan race," (6:896) it becomes clear that this poem is anything but a piracy of Homer's epic. Anchises shows Aeneas the glorious future of Rome and its leaders all the way up to the reign of Augustus who is fated to bring about the height of the Roman Empire. This fate is the reason for the Trojan War and Aeneas' journey, a fate that Homer could not have possibly known during his time but is able to be explained by Virgil now that it has come to fruition during his lifetime.