NO REACH SPOILERS PLEASETHXKBAI.
I have my own theories about Reach, we all know how it ends though. Standard example of dramatic irony.
Reach apparently falls, and the covenant come in and blow shit up. That's not hard to believe since shit is know to BLOW UP, as blowing up is one of shit's most well known properties.
Of course the "new addition" shown in the trailers is "Master Chief." Though he may not be at his current rank, it is the same person. That's pretty much a given I think. The fact that the recruit was the only one of the entire Noble Team to not have his/her helmet off is pretty much a dead giveaway. Also same armor lol.
Some people may have an issue with what I'm going to say next, but I don't really care. I DON'T CARE ABOUT THE STORY IN HALO GAMES, I JUST WANT TO SHOOT ALIENS IN THE FACE.
While I won't deny that storyline helps a game, I really only play Halo to "play Halo." I just want to play a game in which I can shoot an alien in the face. Isn't that why NASA was started? Hasn't it always been man's dream to find an alien race and splatter it's brains on the pavement/moon crater/vacuum of space?
Story should be on the backburner, moar alien brain splatterin' for me please! :3