It is common knowledge that Halo 4 is the direct sequal to halo 3, and the introduction to a new trilogy. This brings to mind obvious changes that will undoubtedly occur, like the absence of the covenant(and probably John questioning his situation.), the an assumed lack of the flood(as it was stated a new enemy would be the antagonist.)
Getting to my point, a topic that seems to have been overlooked by the community is cortana, and her relative sanity.
Normally, a UNSC AI has a normal lifetime of ~7 years. After which they are “decommissioned” to avoid rampancy. Unfortunately, cortana has been active for well over the normal 7 years, even having had some information erased to maintain her stability. Indicators of rampancy have been shown in every game. Particularly rage and meloncholy, two of the most destructive.
It can even be assumed she was already rampant during the events of halo 3, due to the cortana moments and during the later levels of the game, in which she is shown being corrupted by delta gravemind.
This leads me to believe she is either completely rampant in halo 4, her melencholy or rage being intensified by the long(?) time floating in space to the halo 4 planet. Which sadly, seems to be large and full of technology. a requirement for a rampant AI to achieve metastability(sanity, humanity)
So theoretically, cortana may spend much of the next three halos trying to achieve metastability, probably trying to kill the chief during the game as he is the only living representation of her creators, whom rampant AI tend to hate for mistreating them.
In conclusion, the inclusion of a rampancy storyline makes sense, and would spice up the game immensely with “fanfare” that any fan worth his salt would understand. Thus, knowing the 343 team is a smart bunch, I imaging they already have planned.