This sounds really cool, and maybe don't have Flood players at the start, keep them as non-player mobs, but marines who die become Flood players, and people joining after the round starts become Flood as well.
The climax of this round could be that the marines have to abandon their defences to go to the Pelican, and as they're escaping the Flood begin to swarm in and surround the Pelican.
The climax of this round could be that the marines have to abandon their defences to go to the Pelican, and as they're escaping the Flood begin to swarm in and surround the Pelican.
