I've been going through it frame by frame, and several key things have stood out:
The city is New Mombasa, it's written on several walls.
There was an EMP blast.
The drop pods look like they are UNSC, and have that curved triangle marking that appears on a lot of UNSC gear/ships/etc.; This implies the Helljumpers, a.k.a. ODSTs.
It says Halo 3 at the end, and if you manage to pause it at just the right time after the 3 flickers for the first time, you will see the very faint image of the number 4 for a split second. I tried taking a picture of it, but it wouldn't show up for some reason.
My opinion?
Halo Third Person Shooter. Which is why they put Halo 3 in a lot just to mess with us, but the 3 in the trailer is referring to the fact that it'll be TPS not FPS.
It'll star the ODSTs as the main characters, and it seems to take place during the events of Halo 2, because I'm pretty sure that the explosion in the trailer is the Prophet of Regret fleeing Earth as he did in Halo 2.
That's all I've got for now.
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