It's possible. We don't know it until 343i releases some information about the campaign. I think that having the Forerunners as enemys however, is nonsense. The war was ended and there have been no conflicts between the humans and the Forerunners after the Human-Forerunner war. The only Forerunner who might be enemy is the Didact because one of the terminals from Halo: CEA says that the Didact might be very angry at the Reclaimers (the Humans) and that the Librarian is the only one who can temper his rage.