Unknown yet, But whoever he/she is they look badass 

My thoughts have come to some idea's though: 
Chiefs new armor as he is due an upgrade as his armor (as of halo 4) was still Mark VI that he got in halo 2 which is out of date by now. 
or a New playable character in campaign (going back to halo 2 campaign/ODST Switching characters depending on mission) 

either way if its not John they will play a very big part of the story seen as they are the center focus of the image 

I could be all wrong, but as a life long halo fan what i said is plausible