It is confirmed that there will be a 4 player co-op for the Singleplayer. So you will have Master Chief joined by his old old squad blue team. So there will be a total of 4 spartans/players. Also goes the same with Agent's Locke's team.
But due to high graphical and lighting. There won't be split screen for the sake of maintaining 60 fps.
Also DLC's will be free. 343i believe that paid DLC's will split the multiplayer community. Depending on who buys or does not.
There will also be a new enemy.