The enemy could be the Forerunner machines themselves...like mendicant bias. maybe they went crazy bc of the flood control. Maybe the forerunner race is trapped by their own machines. Unless it has time travel the only main chars left are Lord hood, MC, Cortana, and the arbiter...kinda sad rele. but im sure they will have something new....maybe a forerunner buddy? Maybe like Aerrowflex said u will team up with other spartans. Maybe the enemy will be the remaining San Shyumm (prophets race). Im hoping to c the ship the flood came in on. maybe the original vials. I use to hate the flood. but now i love to hate them. they are an interesting specie.