I do believe that we will be fighting the forerunners. And I have to reasons to support my claim.
1. They have said that in halo 4, the covenant is breaking up, and will be scarce. I have heard nothing even mentioned of the flood yet. That leaves one most likely enemy: The forerunners.
2. In ancient times, when forerunners were a thriving race, they were enemy's with humans at that time. There used to be no evidence of this, because when halo fired, the ancient humans were left completly unprotected from the blast. Therefor, wiping out our ancient ancestors. Surviving forerunners, however, would remember this lasting rivalry, and will likely attack humans that they come in contact with. Making them, obviously, the enemy.
Now, as though my points are mostly completely true, I'm am not positive whether or not they are the enemy. I am just pointing out, that if they are in the game, they likely will be.