Open world Halo game gives us a beautiful path to free roam and relax especially if the landscape is beautiful. However, with an open world to just roam leads nothing exciting.. like No Man's Sky. The only thing that would be good for Halo 6 would be the missions. And you know what missions lead to? RPG game.
RPG games have beautiful open world maps to go through and roam. What tops that is the missions, the quest, something to go through besides riding around in a car or killing something. Not just that, you also have a loot system, customization, and sometimes choice making. And that isn't what Halo is. Even if all that was taken out, a free roam game alone isn't that complex/unique.
I understand some of the concerns you've raised, but I'd love an open-world Halo myself. Granted, I'm imagining it as the best possible scenario - as a good, original game that expands the franchise into new, fun territory.
But, more specifically, I'm thinking of it as
an open-world Halo game, not as a generic open-world RPG with a Halo-skin. What I mean is that if I imagine an open-world Halo-game the first thing that comes to mind is the uniqueness of the environment itself. I'm not exploring some vaguely European countryside with castles and villages and plains, I'm exploring a
Halo and that's awesome! The Halo ring is a beautiful, fascinating and original concept to explore in a game, especially if its entirely traversable. Secondly, I imagine the mission-structure through the point of view of Chief's character. An open world wouldn't fundamentally change the way his mind works; and so they should build the mission structure of that world around Chief's priorities. So it can't be...I ride up to a village or outpost and someone gives me a fetch quest. It would be akin to what we saw in the tech demo - we see a distress signal off in the distance and it's Marines who have been marooned and we need to bring them to safety. We ride off to save them, maybe get into a firefight together, and then I return them to a protected base.
Take that basic concept and extend it to a wide variety of similarly heroic scenarios that emphasize Chief's altruism and badassery.
Let's say The Chief has an overall mission to save Cortana. Like that's the "main quest" if you will: Chief trying to deactivate the Halo or prevent Cortana from doing what she's planning. What might that involve? I imagine going to various points of interest, like giant Forerruner structures, and fighting bosses and liberating areas of enemies as these big, focuses set pieces (like traditional Halo missions). But sprinkled through the world, between these missions, are natural random sidequests that involve saving Marines, fighting off insurgent enemies, exploring Forerunner caves to gain access to new weapons or vehicles or lore etc.
Think the basic fun of original Halo CE expanded with better graphics and a touch more freedom.
Just imagining all of this from the point of view of The Chief, and imagining a way for an open-world to make sense for him, these are the kinds of missions and structures that come to mind, and I really think it could work, and that it wouldn't be boring at all.