I am in agreement that with all the existing elements in Halo 343 could actually pull off an open-world style game if done correctly. And it does seem to be going that direction. You'd definitely have a lot of people up in arms as can be seen even in just this discussion. However, I really do believe it could work.
People have brought up missions like 'The Ark' from H3 or 'Silent Cartographer' in CE as examples of a 'soft open-world' experience. And while on the topic of the Campaign, those definitely are areas of big scope containing smaller ones that I could see as a vital part of the bigger open-world picture. Now, haven't you ever looked in the background of the missions your on, down on the distant ground below, and thought, "Oh, I wish I could go there…" or seen epic grand scale battle cutscenes from Reach or Halo Wars and wanted to be part of the action? This is where the more traditional open-world landscapes would come into the picture. While on most missions you might be in more traditional campaign layouts, you would occasionally be put in these large landscapes where either an all out war is taking place as you try completing your specific goal or your hunting down a larger than life machine controlled by a huge Lekgolo colony which is made possible only because of this huge new area. Another open-world centered mission would see you flying downward toward it with only your Spartan armor to assist you, and you'd come swiftly down just as Team Osiris did in the opening cutscene of H5, greatly affecting the outcome of the mission experience as you choose your landing spot.
Again, like you said, the untapped potential here is enormous if truly capitalized on!
The whole open-world/campaign system would work much like ODST where your definitely part of the open-world but focusing on missions sets you on a path that feels like a combination of ODST&CE. You could also manually select the next mission instead of traversing all the way to it. This still wouldn't mean you'd get to explore every single possible spot in the area because of the mission guidelines or situation. For example, you could be under a tight time limit like in CE's mission 'The Maw' but after the campaign is complete, in open-world 'mode' you could explore new spots that were previously hidden or unblocked. Most if not all PvP 'maps' could be found in the open-world too and would likely have unique purposes in it. Not to mention Firefight from ODST/Reach could be implemented in a seamless way in the open-world. Anyway, If done right, the fact that all these missions are part of a larger interconnected open-world should hardly cross your mind, or whenever it does it should create more of an awe factor.
Now, there is at least one big elephant in the room and that of course is microtransactions… Thing is, its not so much a question of if but how. And how they handle it will likely be crucial to the game's popularity.
As a side note, one of the more likely methods they'll adopt seems to be a sort of season-pass of at least mostly cosmetics as that seems to be the format every other game, such as Destiny-2, is now implementing. Microsoft/343 seem to be testing those waters with the implementation of it in MCC with Reach items. Looking at Gears-5 might be another good way of figuring out how Microsoft is thinking of handling MTX in Halo Infinite...
For those of you wondering, no, I don't play Destiny 2. Made the mistake of playing the first one till the bitter end...
Now if Infinite is gonna have open-world esque elements then it makes sense to me the various ways MTX could be done. Now this is purely wild speculation on my part based off the unconfirmed notion that the game will be generally open-world but nonetheless here we go.. This is where RPG elements would probably come in as you'd play your very own Spartan, much like you did in Reach or constantly do in multiplayer. Now while you could certainly make progress to upgrade your Spartan's armor, weapons/loadout, abilities, and such, MTX might make this process much easier. However, I think the main utilization of MTX would be to buy cosmetics (sprays, trinkets, effects, patterns) for everything your Spartan uses. I'd imagine you could buy some things specifically for an overblown price or you could buy a previously mentioned overall cheaper season-pass which contains a bunch of cosmetics but have to be 'earned' through experience gained in the open-world and multiplayer modes.
Its not ideal but unfortunately thats what we have to deal with when it comes to big corporations if we decide to buy into their games… If it worked something like this then I'd find it bearable enough so long as the gameplay content doesn't suffer as a result (which would very likely actually be an issue).
Well, I know that was kinda a downer topic to end on but thats all my main thoughts on Halo Infinite's future concerning the potential open-world aspect. Now understand, I've played the Halo franchise since the first game but I don't want it to feel stuck only in its past. Games like Halo should keep what makes them special, experiment a little, and expand upon what makes them so much fun! Keep the franchise from going stale but stay true to its roots. I think thats something most everyone can agree on. Maybe just not on what those roots are, lol.