>Let's work from the inside out
IMO, Halo's strength has always been its campaign, especially when it's taking place on a Halo/the Ark or whatever.
Also, another one of its core strengths is replayability through dynamism. Even if you exclude skulls and difficulty variances, playing the same level in Halo multiple times rarely yields a similar outcome.
>Let's set the backdrop
The new game is a reboot of Halo 4, coyly called Halo: Forerunner. It's set 30 years after the events of Halo 3 on a newly discovered, nascent Halo, which was made AFTER the Flood were defeated. The game's main narrative hook is discovering how and why this Halo was constructed.
The player is in Master Chief's boots. There's no Cortana rampancy crisis bullshit; you just wakeup drifting toward a new Halo which Cortana has discovered.
>You're gonna wanna see this
she says, as you wake up from cryo sleep.
Inbound to the Halo, the player is greeted by an even older Ron Perlman who introduces the player to Johnson's sexy mulatto daughter, and Keyes's chad nephew. Yes, this part is ripped wholesale from Half-Life 2.
The player's landing is interrupted by unknown belligerents, who are thereby IDed as human colonists. The stage is thus set for a game with multiple factions - cue crashlanding.
>Gameplay loop
The game adopts a soft "roguelike" structure, closer to Majora's Mask. The player has 7 in-game days to figure out what the hell is going on before the Halo is fired. On the ring world, they can come into contact with various pioneering factions like:
>Aforementioned colonists
>UNSC peacekeepers
>Rogue ONI troopers
>Covenant separatists (could just be "Banished")
>Arbiter loyalists (friendly)
>Pacifist Prophets trying to uncover secret of "Great Journey"
>Familiar yellow spores...
To accommodate a more open structure, the player gains abilities like it's Metroid. Some light puzzles and obstacle courses should be added on which to flex these abilities. No ability should wreck the 30 seconds of fun