So just going on the popular assumption we won’t be blasting Elite Separatists the entire Halo 4, it has been stated that we are going to see a new enemy: ‘an Ancient Threat’.
Obvious assumptions is Forerunner or Procusor as a threat, since there is no Flood. The whole Forerunner enemy speculation can be thrown out the window since there really is no reason for that at all, meanwhile, there is good evidence to say that the Procusor’s are actually out to do some evil.
Now, many have also speculated that this new enemy will be unveiled at E3. My two cents?
NOPE.
I think they are trying to with the ‘Sovereign style’(See Mass Effect) for the new villain reveal. It’s advertised on the cover, its foreshadowed through the entire opening half of the game, before it’s finally revealed.
My own feeling is that 343 is going to handle it that way. If it’s going for the whole Sovereign route, the new enemy is going to appear as something inconspicuous, like technology or maybe posing as native Xeno life. Then BAM! Main Antagonist suddenly leaps out, explains how great a threat they are, then disappears menacingly. Something that was in the background is suddenly at the forefront and revealed to be the antagonist.