A long time ago... like back when Halo 2 had been out for a year, my best friend Thrash (now online known as THRASH 101)had a idea for fighting the campain enemies in a base like map (yah I know soulds like firefight but we wern't online or ever internet searcher back then) well he had a different idea then firefight but they are about the same.
His idea was to have a large and I mean large, like forge world size, map with 2-4 player base's around the edge and 1 big Forerunner building in the middle... crawling with cove's (If they got low on ppl more would be dropped in, in larger groups, and they'd even have patrols but would mostly stay in the middle unless provoked by players or well the other extra.
all the while, we the players, could team up/or compete to get an objective in the middle (like a flag or destroy a generator with a det-charge) or race/kill the Covenant, all within a time limit. but heres the kicker at some time... like about halfway in the flood show up from random vent or holes in the map in just the infection forms at first and like firefight it just gets worse (there could even be a point/ game type were you end up haveing to team up with the Covenant AI's to fight the flood off). you could even have marines helping your team and they would shoot aposing team marines and players unless you say had a alliance with them (which eather side would break with killing a ex-enemy team member (player or marine) unless you were playing long enouf where the flood was the greater foe)
yah multi-team Players fighting Covenant while both also possibly fight off Flood it just give me nightmares thinking about playing that, just like Legendary solo Library from Halo: CE.