How many open world games can you list as masterpieces?
A few of these are personal preference (indeed 'masterpiece' is subjective) but the majority of them are critically acclaimed. I adore all of these games and would happily argue they're masterpieces in one way or another:
Horizon: Zero Dawn
MGSV
The Witcher 3
STALKER: Shadow of Chernobyl, Call of Pripyat
Fallout: NV
GTA III, SA and V
RDR
Watch Dogs 2 (imo)
Elder Scrolls: Morrowind
Batman: Arkham City, Arkham Knight
Dead Rising
BotW
Dark Souls (arguably open world)
Prey (arguably open world)
Deus Ex: Human Revolution, Mankind Divided (arguably open world)
Ghost Recon: Wildlands (imo)
A couple of them even have no RPG/strategy systems whatsoever - or at least aren't buoyed by that.
Halo has always been a sandbox buoyed by big set-pieces, co-operative play, and tugged along by a tight but simple narrative. Going open world would compromise all of these tenets.
What?
- Big set pieces can happen in open world (cf. Horizon, GTA V, MGSV, Uncharted 4/TLL).
- sandboxes are enhanced by open world (indeed the majority of GTA players barely ever complete one mission because they immediately play with the sandbox. Games like Horizon and Far Cry are enhanced by encouraging the different in-game factions and wildlife to get into fights - imagine doing this dynamically with Flood, Covenant, etc)
- co-op shines in open world (W_D2, GTA, Ghost Recon Wildlands and Far Cry 4 are all unbelievable fun with open world co-op)
The only thing which could be lost is the 'tight but simple narrative' - but as I said, with careful design this can also be achieved (cf. Uncharted 4 and The Lost Legacy). That said, it's something which is only really being explored for the first time this gen.
Current Halo simply can't support a Breath of the Wild style rethink.
'Open world design' != 'Breath of the Wild style rethink'.
There is more than one type of open world.
Meeting CE's potential to me has never meant going open world, but dialing the Sandbox up to 11.
One of the best ways to 'dial the sandbox up to 11' is to give it an open world sandbox with a carefully controlled ecology of enemies, dynamic events and exploration. Imagine enemy presence starting low key but scaling up as you progress and get deeper into the world/story. Vehicles, weapons and equipment carefully gated by story progression. Perhaps whole chunks of the ring inaccessible until you progress more (the GTA III 'locked island' small-open world idea). Exploration incentivised by offering equipment and lore.