Re: Halo 5 speculation (spoilers for Halo 4)
Cortana becomes the villain. Her rampancy/insanity is obviously the cause, and this justifies her turning against humanity...possibly others, including the Covenant. By Halo 6, the final battle will be between Chief and Cortana.
Just a thought, considering the Prometheans were meh and the Covenant have been done already.