So hear me out. Make it so it unlocks once you beat the campaign and have it so you can select your loadout, grenades, and equipment. Once you press the ‘play’ button, you can just drop right in and explore the open world without any objectives to your heart’s content. Bonus points if you’re able to download and use certain mods in this mode and the campaign in general. Also have it so enemies and/or allies can spawn and respawn at certain locations to freshen up the gameplay and to not make free roam so lifeless. Another thing is that it would make the $60+ price tag more worth it since it would add some more replayability to the campaign.
Now I don’t expect 343 to add this to the game at launch but I would be very pleasantly surprised if they somehow did. I wouldn’t mind having this mode added in an update if adding it in at launch isn’t possible.