Lots of speculation going around at the moment, let’s hopefully put some things straight. This is also just light coverage on the main things seen so it’s not a complete indepth analysis of every second that was shown.
The Sphere
Confirmed to be an object known as a ‘Cryptum’, which was the subject matter of the first Greg Bear Forerunner book.
“Cryptums, or Warrior Keeps, were Forerunner objects created by the Prometheans. A Cryptum was a sealed capsule where a high-ranking Warrior-Servant would enter self-imposed exile as punishment for failure or dishonor, involving a state of meditative hibernation known as “timeless xankara”. During this time, the Forerunner would become unaware of the physical world around him, and could remain in a state of suspended animation for thousands of years. Cryptums required an extraordinary amount of force to breach once sealed. They denied access to every species except Forerunners. - Halopedian Article on Cryptums”
“We know where he’s heading.”
“Same place we are.” - Referencing the Cryptum heading towards the Infinity’s predicted crash site.
The Covenant
The Covenant may have seemingly ended with the death of Truth, but they’re still kicking. The elites shown here are likely the loyalists, those who still believed in the Forerunners being gods and still held a dire hatred of humanity. Though the Covenant may have been fractured, it’s likely they’ve managed to reform at least partially to try and better understand how the Forerunners mysteriously vanished, and possibly enact the Great Journey.
The Creatures
Confirmed by Frank O’Conner in a post Halo 4 trailer interview to be Promethean Class Defense AI’s. The Promotheans were the warrior class in Forerunner culture, and the AI’s were left behind to defend the Shield World, Requiem.
Promethean Crawler - Basic infantry and can apparently be summoned by Watcher units. Small, fast, aggressive and capable of ranged combat.
Promethean Watcher - Flying units summoned by Promethean Knights. Capable of generating shields for friendly units and ‘catching’ projectiles and returning them to the point of origin. Will flee to other friendly units if the unit it is protecting is defeated.
Promethean Knight - Advanced infantry unit with both ranged and close quarters combat abilities. Also capable of short distance warping capabilities. Very aggressive and dangerously armed, will warp in quickly and attack in close if constantly threatened by enemy fire.
Unknown Promethean Knight - Apparently upgraded version of the regular Knight unit. Has a slightly different helmet, a dedicated energy melee blade, and more individual features that make it stand out from regular Knights.
The Weapons
BR - Like usual, but with evident recoil and a visible reticule spread.
AR - Also like usual. No apparent changes from the Reach variant. Bloom seems the same.
Lightrifle - High powered precision Forerunner weapon. Fires what appear to be sniper-like energy rounds. Capable of an optical zoom. Disintegrates enemies on death.
Scattershot - High powered Forerunner Shotgun. Fires pellet style energy rounds. Disintegrates enemies on death (as seen in the Wargames gameplay trailer).
The Visor Mode
Could potentially be the ‘Promethean Vision’ ability that was revealed earlier. A sonar like pulse is sent out and highlights threats in red, whilst also diminishing overall visibility and environment detail.
Cortana
As she mentions, she’s been in active service for 8 years, however AI’s deteriorate after 7. We can clearly see that she’s evidently going through the Anger phase of Rampancy nearing the end of the trailer.
Vehicles
Returning:
Mongoose, Warthog, Banshee, Ghost
New:
Cyclops Mk 2 (could just be for show and not pilotable), unknown all-terrain style vehicle/mobile command center (possibly an upgraded or new model of Elephant, likely just a new vehicle altogether).
The Didact?
It appears to be the case, however the symbol shown is slightly different from the Didact’s in the Halo 3 terminals. Source: Didact H3 Terminal emblem.
‘Ancient Evil’ however seems contradictory to the Didact’s personality, and could refer to another Forerunner Promethean, the Master Builder Faber, creator of the Halo array, and co-creator of Mendicant Bias with the Didact. He has a hatred for humanity, and was dead-set on activating the Halo array. This possibility seems like it would fit the situation better rather than the Didact turning evil.
Rampancy/Factions
The Promethean AI’s have both blue and orange/red colourations, indicating that some could be corrupt/rampant or there are specific factions at work here. It remains to be seen if we may possibly have Promethean AI’s as allies at some point.
Going back to the supposed Didact emblem, the fact that it’s upturned represents the inverse of the Didact’s character, so bad for a better word. Still, it could be another Forerunner entirely who’s gone bad, but until confirmation otherwise, we can only really assume it’s the Didact based on what we already know.