Make this into Invasion. Spartans drop, and you're split into squads of 5. Each squad has a target to neautralize, or take over. Whne a squad captures a position (Could be a building), the install a program into a computer in the building that puts shields on the door, the enemy can't go in it then. You can't stay in the base too long or you start losing health. Only about 15 or 10 seconds. You can then help the other squads neutralize targets. You will also be fighting squads of Elites who have been asigned to defend targets. But, if you drift too far from your squad, the will be ordered to kill you, you will be warned first.
So once you do that, you group up into 3 sqauds. 2 squads of 7 and 1 squad of 6 for each team. Keep in mind you can call in Pelicans for assistance the whole game. So next, each squad will be selected to destroy a Covenant building (Hangar for aircraft, Vehicle Depot, and Weapons armory). Keep in mind that the Elites can take any of these and use them, the doors only open for them. Each squad will be given a Bomb. Go and blow your target as quick as possible and help the other squads.
In the Final round, it would turn into a Sudden Death sort of thing. You must assault The Covenant base. But, everyone only has one life. Once half of the Elites are dead, an AI on the Spartan team comes with a bomb. Protect him until he plants the bomb. He is an ODST, so he isn't as good as a Spartan, and he doesn't fight well. If he dies, pick up the bomb and plant it. Once the bomb detonates, the building will collapse. Escape to the Pelican (If any Elites are still living) and you win.
This would have 30 minute games at the most. It gives a lot of cR as well. It would be very fun to play, but I don't agree to having an armory to buy weapons. They will be on the map already to still give that Halo feel.