Halo is effectively a military sci-fi action movie, its not meant for an open world game, which tends to be slower paced. Its not about a wanderer who walks between settlements of non-combatants doing minor quests for them. Its about a super soldier with a mission. You interact with other military personel almost exclusively, they shouldn't be sitting on their butts giving you side quests, they should be helping you take the fight to the enemy.
Open word game worlds are inherently actually small. To cut down on dead space and let developers focus on what matters, open worlds are small compared to the scale of halo's stories. Heck, between the 2nd and 3rd levels of CE chief travels a longer distance than is possible in any current open world game. Open world would oddly enough completely destroy the scale of halo.
Halo CE had balanced openness with a sense of urgency perfectly.
True, and I'd add: marines perform abysmally when facing enemies (as opposed to facing you when you aggravate them). The two common ways to rectify this glaring issue have been to bestow onto them more health/armor (as in Halo 2) or to increase their numbers.
Reach introduced the concept of marine fireteams that would follow the player after encountering him, but it was mostly in theory, limited to five soldiers, and they often died too quickly. Not sure how H5 implemented it. In Halo CE (Halo, TnR, AotCR) and H3 (The Storm), there were situations where human NPCs who had been saved would progress alongside the player and you could ideally gather up to 16
of them. This would be a way to increase their numbers without giving them an overwhelming advantage from the onset, since it'd be optional to rescue them. But then don't make it near impossible to rescue them, as is the case with the stranded marines on 343GS and Floodgate. More like the ones you find on Infinity.
Of course, devs could actually try to improve marine AI to perform better, but marines shouldn't outshine Chief so the Bungie philosophy of keeping them dumbed down and barely tolerable is somewhat understandable. H3 Arby and HR Noble Team are examples of intrusive NPCs (although I liked Arby). On Nightfall, Jun completely destroys any chance at stealth. I believe there's a way to prevent him from alerting everyone, but the player shouldn't have to go out of his way to arrange this. Commands to stop them from making off with vehicles and charging senselessly into battle will likely circumvent their deficiencies, but I haven't actually played H5 so I don't know what Blue Team is like.
(while still being linear), noted as HCE missions Halo and TnR, seem to work better than an open world
scenario. Even H3 missions The Ark and The Covenant furnish good examples.
More open levels like Halo: CE would be good, but I don't think I would want an open world. I wouldn't mind gigantic levels with multiple objectives which a player can approach in any order that they prefer. Think the mission objectives of Halo: CE's Halo or Halo 4's Shutdown, but on a level the size of Forge World in Halo: Reach. A hub world like Halo 3: ODST's Mombasa Streets might also work.
But creating a Halo game with a Far Cry-like open world? I think that would be really limiting for Halo Infinite's gameplay and storytelling.
Shutdown seems to get overlooked due to the shortcut (although from a speedrunning standpoint, it's much appreciated). The easter eggs (power weapon spawns) were largely irrelevant even when playing through the game normally, since they weren't really needed to even the odds. I think there'd have been more urgency on that mission if there had been marines or even Covie separatists stranded in those towers (optional rescue, of course).
Mombasa Streets managed to make repetitive environments and backtracking work (a common complaint for HCE missions), although it lacked the actual stealth element.