For 343 Industries' first Halo game, if I was to dream something it would be a complete chronicle of the war (First Contact on Harvest to Halo 3's "The Covenant") from an Elite's perspective.
Joining the ranks of Thel (the Arbiter) and Rtas (Spec ops commander "Half-Jaw") would amazing, along with secret meetings with the Prophets.
I've told many people about this idea, and they all suggest I should do a fan-fiction. The secret meetings with the Prophets could be amazing; a deal almost similar to that of the CIA, taking on missions that should be kept in the dark from the rest of the Covenant. One character not enough? Even playing as a Brute could work. It would be interesting indeed to see some sort of friendship be built up between an Elite character and a Brute character, only to see it torn apart by The Great Schism. I'm not asking for any "buddy" type deal, but interlocking storylines for sure, their "friendship" could even been shaky as hell with some minor sense of companionship. This would have to be really played carefully though, because the slightest eye-to-eye relationship between a Brute and Elite would be difficult to sell.
Another thing that would be incredibly interesting would be being alongside people like Sesa Refumee (Heretic leader), and having to see this player fight the urge to join him one the Heretic rebellion begins. This would easily be the most morally dramatic and deep story told so far, with the opportunity for one hell of a character. I know this would never happen exactly as I write it, but this would basically be my dream installment.