Halsey is armless in the shot of her walking next to Palmer. The fact that the cinematography in the opening sequence places Halsey's back to us as she stands in front of a giant blue light heavily supports the idea that this is a flashback. After all, which dead glowing blue person did Halsey spend time with when inside a ONI scientific facility?
People have speculated this for quite some time now; 343 has repeatedly emphasised that they wish to maintain the player population from one game to the next. They've also gotten rid of all paid map DLC packs, and Microsoft probably grills them to still get revenue, so how else to keep the player population as well as get tons of money by releasing expansionary DLC each year that continues the story of Guardians? Halo 5: Different Subtext Here could be releasing each year, focusing on the narrative, up until Halo 6. Similar, in some ways, to Destiny's model of having a comet release every other year.
____
Speculation time! We've gotten a TON of info on the game's story, while still maintaining secrecy over the real endgame and who the big bad even is. 343 has cleverly focused their marketing on the Hunt, pitting Chief against Locke, and by doing so they've never needed to do 'An Ancient Evil Awakens' this time around.
But I think we have enough information to start placing events into a timeline, following a three act structure.
Inciting Incident: Well, for the entire world, this has to be Guardians' awakening. People are dying, worlds are getting attacked. Big stuff happening - and that's what initially starts Fireteam Osiris' mission in Kamchatka. But it could also be Chief's vision of Cortana in Blue Team - 'The Domain is Open ... Meridian is next' - clearly this is the inciting incident for Chief going AWOL.
Plot Point 1: Well, this occurs right at the end of act 1, so I'd expect this to occur towards the end of Mission 4 or 5. My guess is that at this point, both Blue team and Osiris are in Meridian, which we know is full of Forerunner artifacts (If not for that planet, Regret would never have found Earth so quickly). That shot where Chief spots Osiris, then immediately hurries his pace? The sequence where Fireteam Osiris hops through portals and parkours across bridges in a massive Forerunner chamber filled with an orange glow? Locke confronting the Chief and that ending in a fight that cracks Chief's visor?
That is plot point 1. After all, both lead characters are making a solid decision that dictates the consequences - Chief doesn't go back to the UNSC, Locke goes on the Hunt. This leads right into Act 2, the act of conflict; Blue team and Osiris are now actively against each other, and the stakes must be rising if Chief decides that to save the Galaxy he needs to be alone. And supporting this idea would be cracking the visor - it's heavily symbolic and I think it ties in exactly with the first plot point.
My guess is that the portal leads from Meridian to Planet Fungus, what with the weird, giant fungi we see surrounding the lush green Forerunner surroundings. It is here that the Guardian teleports in through slipspace, and here that Chief meets the Warden Eternal - and does not engage in combat. We see this in the Believe trailers.
Following the three act structure, I would place Behind Enemy Lines and Sunaion Station (CANNOT wait to play in the Battle of Sunaion) right before Plot Point 2.
______
Syd Fields' three act structure isn't the only thing we can use to pinpoint the narrative events. We also have Joseph Campbell's Hero's Journey Monomyth:
And again, I think we can place Halo 5's storyline to fit with this structure. We have two lead characters - Chief and Locke - and I think both their character development will fit, although we don't have the entire story yet (So I'll just put down what I think fits, and what seems to fit with the Hero's Journey so far)
The Call to Adventure: For Both Chief and Locke, this initially begins with their respective missions - Retrieving Dr. Halsey and Scuttling the Argent Moon, respectively.
Meeting the Mentor/Supernatural Aid: Locke meets Dr. Halsey, the character who knows how to stop the Guardians. Meanwhile, Chief has a vision of Cortana, who tells him that the Domain is open.
Trials: Well, the fact that there's a music piece in Halo 5 called 'The Trials' shows how music tells a story too, huh! Anyways, for both Locke and Chief they have the Covenant to contend with, an all new Forerunner threat, but at the same time they're also starting to understand that they oppose each other. The Hunt begins.
The First Threshold: Well, this links back up to Plot Point 1. Here, both characters make their decisions. Chief, for the first time ever in his whole life, openly questions and goes against his military indoctrination. And Locke decides that he MUST hunt down the Master Chief - the saviour of Humanity, and Hero of the UNSC. Both are tough decisions; and, with the Chief, there's a literal threshold he crosses as he approaches that big portal we've seen.
At this point, they've entered their Special World - the Unknown. Chief has no idea how to function without the UNSC - he has no intel, no backup, no fallback location, not even Cortana. And I think we can tell from the voice acting that there's a descent - I've never heard Chief sound as 'dark' as he appears in the Believe trailers. Approaching the inmost cave, meeting the god - My guess? These are the Forerunners - the Warden Eternal, to be specific, who begins to unveil and unravel more of the mystery.
So far that's all I've got. If my ideas are correct - then it also shows how well 343 has marketed their game, as I'm super hyped after having seen only tidbits of footage from just the first act of the game, and a little bit.