By the power of truth, I, while living, have conquered the universe.
Let's face it, Halo 3 had a mediocre campaign. A very mediocre campaign.
It starts off and everyone's like "whoa, what the hell happened there?. There's so little room for explanation of how events from Halo 2 led to Halo 3, Bungie can't have expected the majority of the fanbase to have read or ever heard of Halo: Uprising (which only filled in a portion of the gap).
The first few levels of Halo 3 were extremely lackluster, Sierra-117 may have looked pretty but it was pretty poor. It did nothing to tie in events except Johnson saying "the Arbiter is with us". A lot of fans joined when Halo 3 was released, they already haven't the foggiest clue about what's going on, though that's partially their own fault for not looking into the story first.
Anyways, in terms of level structure, here's how I'd have made Halo 3.
- Level 1: Escape from Delta Halo - revisiting Installation 05 from Thel's POV. You're with Johnson, Miranda, Spark and a squad of Elites and you are contacted by Rtas (who has captured a Phantom and is going to "take the Cruiser back"). You are picked up and the objective is simply to take back the Cruiser and head back to Earth, an explosive opening that ties in with Halo 2's ending.
- Level 2: Dreadnaught - John is aboard the Forerunner Dreadnaught, basically the mission would be a revisiting of Halo: Uprising. You fight your way to Truth, you fail to assassinate him and end up jumping out the Dreadnaught leading to the opening of Arrival.
2 missions. That is all Halo 3 needed to make the first part of the game better and the story more enjoyable, even as DLC I'd have brought it. They scrubbed Thel's story from Halo 3 (something I persnally loved in Halo 2), this 1 thing could have been a little nod to those fans.
[Edited on 01.20.2011 2:02 PM PST]