Aye, I've been looking more into this/reading others opinions, But do you notice how the other spartan, the body shape/size is like that of a female Spartan (bigger chest plates and smaller waist) Though a tweet from Frankie O’Connor stated that its not a girl but he could be just trying to throw people of that lead. 

Or some others have said that you maybe to play as another spartan alongside the chief with reaches option to have your own spartan in the campaign. 

And also looking close at the image the helmet reminds me a lot of the Rogue helmet from halo 3 multiplayer, so we could possibly facing a rogue outfit of spartans or insurrectionists that got themselves some armor?