(More info and edits/updates will be under the dotted line)
OK, bear with me people, this may be a very long read.
I was sleeping last night and right before I went to bed, I was reading the Halo: Enclycopedia. I had also played a little bit of campaign in Reach and was thinking of how the campaign wasn’t as great as Bungie made it out to be. Don’t get me wrong, the campaign is very good and a fresh new experience but the whole thing about “bigger battles” well… i feel that bungie could’ve done better in that area. Instead of taking part in the epic fights for the orbital platforms and castle base, we only fight in small battles around Reach (With, of course, a few exceptions). But I had a dream where I was a marine in the battle for Harvest and hundreds of marines and covenant, on each side, were fighting each other. (Yes, I know, I’m very sad XD)
But of course this game would probably be impossible to make, due to engine limits and stuff like that. I do know that there is a PS3 game called “M.A.G.” where there can supposedly be 256 players in the same multiplayer game, but I don’t know if a halo game would be able to support that many players. 256 players isn’t the goal of this game, the goal is to make the battles feel like those in the book. In my visionary game I imagined you, the player, fighting as the UNSC or Covenant on lots of Colonies and locations in Human and Covenant space as well as some locations in Forerunner space. All of these locations would be from all walks of halo canon, some from games, but alot of them from the books of the halo universe.
I would like to focus alot of these locations on the books because many of the stories in these books are very important to the overall story in halo. Such as the battles of Sigma Octanus IV, Harvest, Jerico VII, and many more colonies. All of these locations are important to the halo story but are never mentioned much in the games. That’s the other reason that I came up with this idea.
“How would the game work?” You will play as a marine or army trooper who fights in all of these battles in different theatres of the war. As the game goes on, you witness the eventual fall of the outer colonies to the covenant and try to halt the covenant’s progression to the Inner colonies and Earth. You fight in many battles, huge and small, and during these battles you will learn alot about the many different locations in the halo universe with every colony having its own range of enviroments and inhabitants, with some being civilians you need to save and others being insurrectionists who are sometimes hostile to you. Also, alot of weapons, vehicles, AA’s, equipment from both all the games and books would be used depending on where and who you’re fighting, many of them having never been seen before.
As for multiplayer, it would be alot like past halo games, but alot different, depending on what game mode you choose to play. This game mode that I’ve been thinking alot about will be refered to as “Invasion 2.0”. Because that is what it is. But it also isn’t. With these new large scale maps and vehicles, battles will be bigger and better. For example, there’s a game of… lets say 100 people fighting inside the city at night in Halo 3:ODST, 50 marines against 50 elites I guess. The whole nighttime city is huge, right? So for to get across the huge battlefield, the map would be filled with vehicles to get from point A to point B. Maybe give each team a pelican (with other vehicles on the ground of course) that holds 11 people (Pilot included), has a front chain gun and can carry 1 warthog, 1 scorpion, 2 mongooses or 4 weapon pods to help teammates on the magnetic thingy on the back (Carrying different and/or more stuff will slow it down and the enemy can’t steal your pelican). The pelican, being a big target and all, will need quite a few rockets/lazers to bring it down. And if you’re a pelican pilot with 10 other dudes and a scorpion hangin out the back, be dang sure that the enemy will be gunnin for ya. This is just a sprinkle of changes that gameplay would have, with other changes including the fact that you’re marines and not super soldiers, new toys, levels that make Boneyard look like a flea, different types of levels, ect. All this stuff is too much to write in.
If you want specific answers to your specific questions, then shout them out in the comments and I will get to you as soon as possible. Tell me what you like/dislike about ths idea. Thanks and hopefully 343 will see this 
And sorry for any mistakes, I’m new to waypoint so… yeah.
EDIT 1: The game could be made with Reach’s engine (seeing as 343 is using the reach engine to make Anniversary’s multiplayer). It would have rare types of characters that would be more powerful than others and have different skills and abilities, such as Spartans being more powerful than ODST’s being more powerful than marines. Same is true with the covenant. The game would focus on HUGE levels with vehicle-based combat on some but not all maps. Gametypes would include Invasion and Invasion slayer, Team slayer, Free-for-all etc.The health system would be like the stamina-system in ODST, except for Spartans. Only certain game-types would support adding stronger characters
as the game progresses.
Customization options would be exactly like Reach but this time you get to add voices to your marine/trooper just like firefight. Plus, in Halo 3, when you brought up your service record in-game, you saw a “portrait” of your player. This would be brought back and you can add backgrounds, vehicles, weapons, etc., into this picture to truly customize your player and have a VERY unique online identity which goes further than armor and colors. You can only do this to your UNSC and covenant marines/troopers and grunts, not any other characters. (For gameplay reasons) And you would be able to have a whole variety of Halo music from ALL the Halo games that would play in games. You could buy this music with credits and then record certain lengths of the music and put them in your own custom games. Plus, this music would play all throughout matchmaking games just to add to the epicness.
Note that the purchaseable armor pieces would be for the marines/troopers and grunts only. I don’t know if the ODST’s and Spartans should have unlockable armor like the Elites in Reach. And the objects in the player’s “portrait” would be free but would have to be unlocked by ranking up.