Halo as a Latin Epic

I like where the thought is going in this small piece from Joystiq:

To what extent is Halo an extension of the interactive storytelling of old? The latest issue of The Escapist has a thought-provoking piece comparing Bungie Studio's Halo with Virgil's The Aeneid. The theme of a revered, super-human warrior protecting his home world against two enemies (one with whom we can sympathize, in the case of the Covenant / Greece) is prevalent through each tale.

The interactivity, according to author Roger Travis, is an illusion for both the gamer and the Roman audience: we are immersed within the story but have no say over its outcome. Master Chief as Aeneas notwithstanding (no word on where multiplayer fits into the comparison), to what extent could one design a game where the progression is wholly determined by the user?

Could the lore of World of Warcraft be expanded through the present-day actions of its masses? Even open-ended games like Fable and Oblivion have main quests and endings. To play devil's advocate to our own question, perchance a central plot is required in story-driven games in order to give the wandering avatar an initial feeling of purpose. Could there be a cutoff point where the user is left to his or her own devices, or must we always be in pursuit of some tangible outcome laid before us?