The worst part about Firefight for me was the AI and its limited capabilities. It was very dumbed down from the normal campaign AI and they lacked any kind of dynamic behavior whatsoever - their sole directive was to advance to your position without stopping until you killed them. If firefight were to return, the AI would need some serious beefing, as well as the implementation of more gameplay dynamics and larger maps to incorporate their improved behavior.
I think a good place to start would be allowing the AI to set up defensive positions of their own. Find areas in the map to take cover behind, deploy stationary guns, mortars, and artillery from, dispatch small teams to assault your position, and a number of other scenarios that would make it feel like an actual battle, rather than wave after wave of dumb enemies. Their positions could even have the capability of growing in size and power, such as starting out as a simple machine gun pit, then growing into an encampment with key leaders (Elites, Brutes) in charge, to a full-scale firebase with sniper towers and other structures that can spawn enemies directly, or serve as a drop zone for more troops and vehicles.
This could even become a secondary objective of Firefight mode - stop enemies from setting up a significant presence in the area, or in failing that, assault their firebases and wipe them out / destroy them. As Spartans, the same options would be available to you as well, allowing you to set up defensive positions during lulls in combat and call in ordnance drops or personnel. This would create the potential for drastically different outcomes each time the game is played, all depending on how you want to play it. You could stay on the offensive and seek out enemies directly before they have a chance to mount a counter-offensive, or build up your defenses and duke it out with an enemy base in a battle of attrition. Or even lay low for a while, letting the enemy set up a few spots, and launch hit-and-run raids.
At this late point in the development cycle, its too late to add something this big if it doesn't already exist, but there are still future Halos and I think this would improve the experience drastically.