I was thinking that an ODST squad (comprised of 6 people, heavy weapons, explosives, sniper, leader, close quarters, and medic) would land on a Halo ring that's uncharted. They were dispatched to collect information and locate the cartographer, but then they disturb the hive. The Flood becomes active, as well as the Sentinels, who would kill both the Flood AND the marines. At some point a contingency of Covenant land, who end up helping the marines in the end, although they fight against each other for a while. The marines end up with the date, the Covenant survive, and the ring is destroyed. Everybody is happy. That's the gist of it.