Beginning:
CE, 3, Wars, Reach and 4 have similar starting missions. The player is familiarized with the game over relatively docile locations, and this is when the game's sandbox is slowly introduced to player.
Twist
Ah, what a twist! In CE it was the release of the Flood, which changes the gameplay, level design and atmosphere for the game. Ditto with Halo 3 and the Portal revelation, although the Ark/Covenant are the exceptions to the rule as they are the sandbox missions of Halo 3. Either way, after the twist, the plot accelerates rapidly. In Halo 3's case, 80% of the plot involved events after the mission "The Storm". In Halo Reach, the arrival of the supercarrier changes the Campaign from one about random missions targeting Covenant advance groups, to a story about desperation and survival. (Big battles was never really a thing, although the start of LNOS was nice). In Halo 4, the twist comes slightly earlier, with the awakening of the Didact. In Halo Wars, it involves the kidnapping of Anders.
Either way, the plot in each game shifts into the correct gear 40% of the way through.
Climax
The final section of each game involves three-four sided battles, and the story is concluded.
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Halo 2 and ODST were different. ODST had a non-linear campaign structure, although I'm going to concentrate on Halo 2 here.
Halo 2:
One of the reasons why Halo 2 is my favorite Halo campaign is due to the "Act" structure of the Campaign. Significant elements of the story are present throughout the game, ranging from the attack on New Mombasa, to the death of the Heretic leader. Basically, Halo 2's plot isn't all concentrated at the end of the game
Halo 2 can be split into 3 acts:
-The Heretic to the end of Metropolis. Opening act, introduces the Covenant, and sets up the story for Halo 3 ODST and Halo 3's first half. 1 cutscene, 3 MC missions.
-The Arbiter to Regret. Shortest act, but two notable characters die; the Heretic Leader and the Prophet of Regret. (With two bad boss fights). Their deaths are very significant to the rest of the plot. 2 Arbiter missions and then 2 MC missions.
-Final Act; the largest act for the remainder of the game. Full galactic war, the board is set and the pieces are moving for the next game.
I'd speculate that Halo 5 might actually be something similar.
-It will probably start off with the MC doing whatever the fuck he wants to do.
-Locke follows, twist in Act 1 to get the wheels moving.
-Act 2 involves 1 or 2 significant events which setup Act 3, and perhaps even the next Halo game as well. Locke and the Chief will likely meet at one point, and perhaps even go their separate ways to fight the battle on two fronts. I wouldn't be surprised if the Chief ends up fighting the Flood, while Locke deals with the Covenant/Prometheans. Expect multiple three-four sided battles later in the Campaign.
-Act 3 involves full galactic war, the board is set and the pieces are moving for the next game.
