There’s really nothing stopping me from saying that Halo 2 was an immaculate game,
however, I believe there still is some room to make it even more interesting. Does everyone remember the second to last level High Charity, the last time we got to see master chief until another three years. Anyway, I know for a fact this level wasn’t explained in full depth. I mean, Master chief is one human, out of how many millions of living creatures on this moving dwarf planet? An in depth struggle of alliances or every grunt for himself, or last Phantom on dock B, Who will get it Elite honor guard and his wounded brother, recently hired Jackal contract snipers, Grunt heavy team, or other dozens of combinations that would have been great to play. At least, make an awesome cutscene. Perhaps, there’s an elevator shaft with Brutes and Elites working together to move supplies to their escape ships, Elevator isn’t there (we all know why). One Elite sighs, “If only we were still the security around here”. His brother tries to start another topic to keep them friends and allies, as we know Brutes aren’t the kind to ignore a confrontation. This starts an argument, Flood gets involved. Brings them back to reality. But the fact remains they have to find an alternate route to carry supplies.
Why is this only revolving around Covenant affairs? It’s not and there’s the beauty of it. Humans, Flood, and my name description for engineers, Slaves and independents.
Halo 3: ODST went into very good detail of the Engineers capabilities. And the fact that there is no “true” aggression towards humans. I want you to picture an engineer, (forced to use their technological skills to manipulate electronics)trying to connect the Amber Clad’s mainframe, when discovered by a few surviving marines, they decide to aid the marines and get some pelicans operational for means to escape the city.
Perhaps some additional features should be added into Halo 2 Anniversary, perhaps a game type called Memory, in which these side stories are featured. Or even a story telling firefight. Perhaps it’s just an idea, for another game and perhaps I am onto something. Regardless, D4RKSTAR2 out.
First and foremost, if they’re going to add anything, they should add the 4 cut missions: Covenant Ship, Forerunner Tank, the Dreadnought mission (the original ending after the entire 3rd act was cut), and the infamous E3 level. Campaign Theatre and a skull menu should also be included. Perhaps extra story components will be added in Terminals.
If they’re going to add Multiplayer, I’d love to see the full Matchmaking system improved, along with enhanced Custom Games featuring all the options from the other games, as well as multiplayer Theatre, Forge (maybe a map editor; there’s a precedent in H2V), and a proper Custom Games Browser. 
They to improve how skulls work in Halo 2 Anniversary; originally in Halo 2, skulls were only obtainable when playing on Legendary, activated as soon as you picked them, and turned off as soon you shut off the console; meaning you had to find them all over again if you wanted to use them again.
There were also very difficult to find, often requiring you to use all manner of ridiculous tricks (like grenade jumping and whatnot) in order to get to them, sometimes requiring two players even. Skull unlocking and selection should be more like Halo 3 and those games that followed it, but with all the skulls that were originally in Halo 2.
They should also bring back the skulls from Halo:Combat Evolved Anniversary, like Bandana (must-have) Grunt Funeral, Recession, Malfunction, and the one that disables auto-aim (I forget what it’s called).
I also wouldn’t mind some new vehicles added to the Campaign missions; specifically, vehicles from Halo Wars. Things like the Grizzly and Locust. I’d love to replay the bridge segment of “Metropolis” with the Grizzly in place of the Scorpion. Or maybe go through “Quarantine Zone” with a Locust. Now that would be awesome. Maybe have a new Skull that enables these vehicles, for those who want to mix things up a little.
There’s also the matter of the Brutes; I really think they should go with the Halo 3 design for the Brutes in Halo 2 Anniversary, and also give them some of the weapons they had in Halo 3; namely the Spiker and Gravity Hammer. Bungie didn’t have time to flesh out the Brutes in Halo 2, they shouldn’t be exactly the same as the original.