I been pleading 343 to bring a forerunner faction, it could be totally possible if CA makes the proper balance and in other halo game we have seen enough for runner units and building in order to create a HW2 faction, and now that HW2 was connected at the end with halo 5 guardians.
So it would make sense to have a forerunner faction, leaders could be: cortana, guilty spark, didact and the librarian. As well imagine using forerunner leader powers to destroy enemy units and create promethean knights from them! That would be awesome, since i first heard from HW2 coming out i imagined that they would include a forerunner faction.