A friend of mine and me have been talking about this for awhile.
New factions altogether, maybe bring some of the originals back.
UNSC
Spartan 117: A lone Spartan (perhaps blue team of three?) that would act like a Covey Leader and be on the battlefield. He increases unit morale, perhaps making all units do a tad bit more damage. Increase the Spartan AI, defense and health to make him last longer. His specialty won't be to hijack, that can be an upgrade however. Instead of having a Covey specialty the Spartan may allow you to call in a Spirit of Fire-like menu that allows the usage of a Cryo Bomb and Scatter Bombs, but nothing major.
ODST Squad Leader: Another unit that actually goes onto the battlefield. Allows usage of another Spirit of Fire-like menu, only to call down ODSTs of different varieties depending on your tech levels. Have a specialize unit for Sniper-Scouting(Anti-Infantry), one for anti-vehicle and aircraft (Grenade Launcher), a specialized Medic (but can still fire a gun), and the ODST Squad Leader. The Squad Leader alone isn't great, but he has an increased AI and when his team is together they all receive a health and defense bonus. If certain members are lacking a smaller boost is given, etc. Sorta implies another morale thing, they see a member die and it's game over. ODSTs are amazing, but aren't quite like Spartans. Obviously there would have to be a limit to how many of each should be made, but what would that limit be? Help me decide! :) I'm thinking a total of three squads altogether, so only three Squad Leaders. I'm thinking only Two Sniper-Scouts per team, five medics, and 3 Grenadiers. All units can enter and leave vehicles.
Sniper-Scouts: Increased range of site when scouting. Does extra damage to infantry and light vehicular units such as the Banshee and Ghost. More durable units such as a tank or armored aircraft will receive very minor damage due to balance reasons.
Grenadiers: Obviously the main infantry of this setup. They have their SMGs and Grenade Launchers to fight with, they will utilize the EMP blast to stop any vehicle on the ground, and any air unit that is susceptible to an EMP blast. The Grenade Launcher will also be used at a range against big clusters of enemies, however if the enemies are spaced out the SMG will be utilized for efficiency.
Medics: They have SMGs to defend themselves with, maybe just a shotgun. They can't heal while fighting, however when the fighting ceases they heal any damaged infantry unit in the area. Does not matter if they are in or out of a vehicle, the vehicle will not be healed.
Squad Leader: Has a DMR/BR. lol His ability as a leader gives him the ability to direct his squad more efficiently as a computer.
The ODSTs will not join all three squads and create some omega bad-ass unit. To counter this the game can just categorize it itself, or you can assign to them specific colors after they are made: Red, Blue, Gold, Green, etc.
Cortana/Any A.I. Such as her: Being an A.I. she herself will not fight. However she has two upgrade paths, you can not have both. One is to greatly increase non-infantry unit production times and research/building times due to her nature, or to hack into the enemy's bases and lower their build/research/non-infantry unit production time. To put this into perspective, if you choose to help yourself it's a 2x multiplier, if you choose to hinder the enemy it's a 1.3/1.4 divisor.
Her upgrades will allow you to further increase the flexibility of the path you chose.
Path 1: The path to help yourself. An upgrade to this will be the ability to enforce an even faster build time for one specific unit such as a Scorpion. Another upgrade is to enhance another unit for speed, power and defense and an increased health bar. Perhaps even a shield generator. This reduces that unit's build time by 2.5x. If you choose to enhance the unit you chose to speed up production of, it will simply cancel the upgraded speed altogether and it will be like you were playing a normal game. Enhance your incoming supplies or increase your tech level by one.
Path 2: Enemy hindrance. Not as many paths for this. You can remove the enemy's ability to use the radar. You can slow their income rate by 0.5x. Anything else would be almost unfair to your opponent, as they wouldn't really be able to build anything.
Cortana will not affect a team as a whole. In a team game you can choose to help an ally or to affect one enemy. The target ally is interchangeable every five minutes, the enemy every 15.
Captain Cutter: Or any other Captain/Commander. Basically this is the one that can use Orbit controls to attack. Please expand on this, we haven't really thought of many variations to keep things balanced for him.
COVENANT
Prophet: Acts like Cortana. Many people did not like the fact that the Prophet got into the fight, as Prophets are not a warrior race. Instead, the Prophet now will hide inside of a base. He can teleport between a base if he so wishes. When playing as the Prophet, all bases receive an extra layer of Energy Shields and special units will receive a decrease in build time. The bases get extra protection because a Prophet is the highest form of Leader they have. If the Prophet's current base is being attacked, he must wait an extra thirty seconds to teleport to a new base. If a base is destroyed with the Prophet in it, the player automatically loses the match. In dire situations, a Prophet may reveal his location by adding an immediate extra shield to buy himself some time for escape. He can control orbital attacks to a degree, such as dropping Hunters down to Earth like an ODST. As the Prophet is in a base, that base will generate a certain type of energy. It takes this energy around thirty full minutes to charge. When this energy is charged, and the Prophet has the highest tech level he can achieve, the Prophet may call in a Glassing and end the game. This energy does not gather if units are being made, if the base is using energy to hold up a shield, or if the Prophet is not in the base. A base can not transfer the energy it has stored inside it to another base. This is to keep things balanced but at the same time, to keep the enemy on edge. Obviously this won't be used much in 1v1, but it will definitely be something to incorporate as a strategy in a team game such as 3v3 and 4v4.
Bad-ass Hunter: Hunters as we know are made from worms. Alien worms. I suggest to you we make a leader unit of Hunters! One that is slightly bulky with more power and more armor, and one slightly less powerful that is a bit more long-range and quick. Most of the upgrades will be for these two special Hunters, further increasing their defenses and attack strength. One upgrade will be to let the Hunters step into a Revenant. Not much in-depth strategy for this one, but realistically Hunters usually always kill a Spartan or two whenever they were encountered in the books, so let's make them an amazing leader unit!
Arbiter: We can't go without an Arbiter. While I really enjoyed the Arbiter from HW1, I don't think I'd like to see a returning one like that in HW2. Namely because the Arbiter in this one wasn't all that smart, just a tough brute of an Elite. I'd like to have an Arbiter that is a lot like Thel Vadaam from Halo 2 and Halo 3. Powerful, has a shield, and has battle smarts. Upgrades include better weapons, allow him to interchange them depending on the situation. One upgrade would be a Plasma Launcher to counter Vehicles and Aircraft and clusters of enemies. Another would be a beam rifle, and then the last one would vary depending on which of those you chose. Arbiter will always start out with a cloaking device and an energy sword.
Plasma Launcher: The Arbiter will receive another weapon to utilize. This weapon will be a Needle Rifle. The Needle Rifle will increase his attack range and he will utilize it in situations the Plasma Launcher would not be necessary.
Beam Rifle: Yes, a beam rifle. Not like in Halo Wars with the Jackals either, mind you. This will be used solely for headshots and long-range missions. Has a slight EMP effect against small aircraft and vehicles such as Banshees and Ghosts. Banshees, if too close to the ground or an object, may spin and crash. Ghosts will not move for two seconds. A Revenant will be able to keep moving, however it will lose it's ability to shoot a blast if the gun is hit. An upgrade for the Arbiter if you choose this path will be an even greater sight increase then what the ODSTs offer, and the Arbiter can use the Beam Rifle to snipe at a greater distance.
Brute Chieftain: Uh-huh! He's back again, and about time too! His coconut gun- owate. Wrong game.
Chieftain in the last game had a pretty unique ability, and I liked it. I enjoyed how he had stuns as well. I can't offer much to improve this, and I honestly wouldn't want to change it too much. Perhaps make his AI a bit better? He was slow as hell to be a Chieftain, and he didn't take out tons of infantry unless you utilized his specialty. Make him have a more aggressive AI like in the actual games, able to clear through normal infantry with ease and even challenge a Spartan in Melee.
Other
Forerunners - Oh yeah!
The Flood - Please, just let us use the flood in one game!
Rebels - Differentiate their weapons or something.
Heretics - A new idea from Reach. Mix it up some and allow a certain branch of the Covenant use Human weapons with Arbiter as the leader.
Building Structures
Alright, we have our normal Infantry, Vehicular and Aircraft buildings going on. But we don't, however, have anything else to sport.
So let's redesign the system some.
UNSC Infantry: Marines, Flamethrowers, Spartan-IIIs
UNSC Vehicles: Warthogs, Scorpions, Elephants
UNSC Aircraft: Falcon, Hornet, Sabre
Infantry Upgrades: Same, Same, New names but same concepts. Spice it up some with upgrade paths, pitch in! New ideas rock! :)
Vehicle Upgrades: Starts with a turret, upgrades to Rocket or Gauss Hog after Grenadier upgrade. Starts with turret and Canister Shell and allows people to ride on it. Upgrades include more people, faster shell reload times, and possibly more armor. Elephants become invulnerable to infantry-based and light vehicle attacks. Still produces units, upgrades include a better engine and increased armor for defense. Starts with all turrets for defense.
Aircraft Upgrades: Turret upgrade, allows the pilot to use a gun. Turrets become slightly stronger. Grenadier turret path is an option, following it up allows remote detonation for EMP blasts as they are not included with just the grenade turret alone. If you choose to upgrade the machine gun turret to a Gauss turret instead, you also gain a bit more mobility. Hornet has the same overall upgrades. Add some ideas? Sabre will be pretty cool as well. :)
Covey Infantry: Grunts, Jackals, Hunters
Covey Vehicles: Wraith, Revenant, Scarab
Covey Aircraft: Banshee, Vampire, Seraph
Infantry Upgrades: Adds an Elite or Brute to the squad, Can go to a Fuel Rod cannon or Plasma Launcher first, last upgrade will be a mixture of both for the squad. Jackals will be given a strong shield up front, you can double the shield's effectiveness, and then they recieve a Focus Rifle. They all start with Carbines. (yes, carbines) Hunters will keep their basic stuff, except lose the beam and replace it with something that helps to buff their health.
Vehicle Upgrades: Wraith will start with a plasma modulator and a turret being utilized by the computer. It can receive the heavy shield upgrade first, it will learn to use a boost which will be utilized to splatter. It can then go two directions; Anti-Air Wraith, taking away some more of it's already limited mobility and effectiveness against ground troops, or it can be given an extra shield. Revenant will start out with two Elites inside. It starts with a boost and can splatter. It's upgrades will be to improve the damage and fire rate of it's plasma launcher, then it will also receive an overlaying shield to protect against sniper bullets and ONE EMP blast. Scarab's now have their own spot and require a bit less to make, however to gain the materials in this game will be a bit harder anyways. It will have people on board and people that, if designated, can get on board, to control turrets to shoot at individual enemies. The Scarab will utilize it's giant fuel rod gun more often as well. EMP's will not affect it. Units on board the Scarab have their own health and can be designated as a single attack.
Then we will have new buildings...
A Hospital for the UNSC, the equivalent in the Covenant will be an Engineering Facility.
Hospital will have, well, medics and other specialized units that will heal vehicles and aircraft and buildings. Vehicles and aircraft can land on a Medical platform that extends to the side and they will be repaired fully without the need of a specialized unit, upgrades for said pad will be available. Can deploy Cyclops battle armor to heal or destroy a building.
Engineering Facility will have two Engineers. Those that heal, and those that protect. Those that protect will give an Over-shield to Infantry and Vehicles, but not Leader types like Arbiter and Chieftain and the Scarab. Then you will have the ones that heal.
Recon Buildings. Not yet picked a name. These units may just go hand in hand with the base itself as it's made. Will Specialize in Scout troops that can scout and defend themselves. Contains normal snipers, aren't as good as the ODST Sniper (and if we throw in three Spartans for the Spartan 117 to make blue team, not as good as that Spartan). Snipers will have increased visual range, half of that range will be within firing distance of the Sniper. Covenant will send a lone Skirmisher with a Beam Rifle out. Mongoose will be made at start-up for the UNSC. Covenant will start with a Ghost or Chopper.
Light Vehicle Building for the Covenant. Contains Ghost/Chopper, Locust and some new vehicle we can pull out of our ass like we did with a bit of the other ones. It's intended to be lightly-armored, for speedy getaways and scouting and whatnot. Nothing with huge power though.
Covenant Specialties Unit: Suicide Grunts, Skirmishers, and Brutes/Elites
Suicide Grunts: Same ***, different game. Their sole purpose is to die. Skirmishers. Almost as expendable as a grunt squad, very low defense and relatively low health but extremely agile, smart and precise. Brutes/Elites count as your main infantry, but a bit better. As a price for being better, they take a little more to make a little longer to deploy.
Upgrades for the grunts will be a stronger and bigger explosion, more running speed, and perhaps a cloaking device for an ambush. Skirmishers will be given a gauntlet to utilize if necessary, Needle Rifles and Needlers, and the final upgrade will be a partner that makes them produce in pairs as opposed to one at a time. Skirmishers are anti-infantry all the way. Elites/Brutes will have their own upgrades and paths they can take. Elites and Brutes can both go Recon invisible, or they can be a war hero. You decide.
Do away with Super Units, maybe. The Covenant have a lot more going for them, we'd have to fix up something for the UNSC. Maybe add more units to them? Do away with the Rock, Paper, Scissor triangle we have going on. Adjust unit prices properly to counter-balance this. Infantry is very cheap and expendable. Vehicles aren't as cheap and take a bit longer to build. Air has dominance over most things, to counterbalance they are the hardest to even obtain. Certain vehicles like Scorpions that possess a very quick shooting time may be able to aim and destroy aircraft, or vehicles or infantry with specific upgrades will be available.
Make maps huge. Absolutely huge. More bases even for 1v1s. Make it 4v4, one massive free for all up with up to eight people, 3v3, 2v2 and 1v1. If a person leaves please replace that person, based on how they were doing so far in the match, with a computer of roughly equivalent strength. Even a horrible partner CPU is better then nothing. May not apply to the FFA, as it's a one man team.
Have some empty, neutral vehicles placed on the map. Allow people to send units to get inside of them. Allow many units, such as leader types like Arbiter and Spartan 117, be able to hi-jack a vehicle. Brutes and Elites as well, for example. It won't happen often but it may on occasions.
Spartan 117: Some additional notes for him. If we choose to have him act sorta like the ODSTs, where he can make a team of three, allow him to pull up the same menu he would normally use to call in a Cryo bomb and a scatter bomb. Have several other Spartans to choose from: Spartan-058 - The Sniper, Spartan-087 - The Runner, Spartan-104 - The CQC Expert, and Spartan-029 - The Electronics Expert. Perhaps we could work in the whole team eventually? Ideas? Thoughts about that?
I have a lot more details but I'm running out of time. I'll probably edit this post or post again with the details and whatnot. Let me know how it sounds so far!