Is the campaign going to be open world
We don’t know yet. In my opinion, no. Not a true open-world. Halo has always had linear story telling, going from A to B to C. It will likely amount to expanded, large missions, with side objectives (or maybe discoverable side objectives that aren’t listed but will result in help in that mission such as freeing a group of Marines). However, my estimation could be completely wrong, and we simply will not know until 343i deigns to divulge that information to us.
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> We don’t know yet. In my opinion, no. Not a true open-world. Halo has always had linear story telling, going from A to B to C. It will likely amount to expanded, large missions, with side objectives (or maybe discoverable side objectives that aren’t listed but will result in help in that mission such as freeing a group of Marines). However, my estimation could be completely wrong, and we simply will not know until 343i deigns to divulge that information to us.
This is honestly my hope for the campaign. I just can’t get into Halo having a huge open world, especially when the only precedent for it wasn’t that great (I love ODST, but the Mombasa Streets were pretty lacking). Something like Silent Cartographer or Winter Contingency, but on a larger scale is what I hope to see.
The Creative Director for Halo Infinite was Tim Longo, famous for the Tomb Raider (2013) reboot, winner of 30 “best of E3” awards and 80 other “best of” review awards including “best console experience”. Tim Longo made that game “Semi-Open World with Hub Areas”, where linear sections open out into larger explorable areas, then into Hub Areas giving a variety of choices for the main story and other side-missions and challenges.
343i seem to find it hard to find words for the nature of the Infinite campaign, not wanting to say it isn’t open world, but not wanting to say it is either - claiming it’s mainly due to people’s understanding of “open world games”. Meanwhile, if it’s semi-open world, they haven’t mentioned “Hub Areas” like Tomb Raider games.
Reading between the lines then, maybe 343i could say it is “Our own take on a variety of semi-open world, inspired by ‘open world’ ideas where suitable for adapting to a Halo FPS game - so you’ll only really see what we mean when you play it, because there’s not quite another game like it!”.
Suffice to say, after Halo 5’s Spartan Locke was “designed to be a cross between James Bond and Lara Croft” (Brian Reed), we now find that like Lara Croft, Master Chief has a Map, a new Silver Magnum and a Grappling Hook - is this all part of “Halo Infinite: Rise Of The Halo Raider”?
Indeed, compare these two scenes, as perhaps 343i got some inspiration from the “crash scene” opening cinematic of Shadow Of The Tomb Raider to create the “crash scene” opening of the July 2020 Halo Infinite Gameplay Trailer…
So to me, it seems all these Tomb Raider references suggest Infinite is inspired by Tim Longo’s combat and exploration ideas for the Tomb Raider reboot games, and suggest it’s “semi-open world”, with or without Hub Areas…
Hope this helps. 
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> The Creative Director for Halo Infinite was Tim Longo,
He was but he left mid August 2019. I would agree with all you wrote if he was still there. Who knows what we’ll get now. I reckon semi open world based on the terrible gameplay demo we saw, but anything could have happened since his departure. Too many development issues with Halo Infinite for my liking.