What I Want for Campaign:
It would be fun to be able to choose certain things along the lines of Player interaction. I've been watching HaloFollower on Youtube recently and people have had really great insight on what should be implemented into the game.
Ability to choose if people can join you
You should be able to choose whether other players can join your game and take the form of the Arbiter, other Elites, Spartans, or even Prometheans. If the Arbiter is returning like people say, then there should definitely be a mission dedicated to his return and the people that join a player in that particular mission/missions should be Elites. The Dead Rising series has this ability, and you can choose people who are in the same area of play style as you. Possibly you could choose someone who likes to Speedrun, or someone who likes to take it slow and look around, so that they can find terminals or the Halo 5 equivalent.
What I mean when I say interactive gameplay is that Brutes, Elites or Prometheans charge up to your character and try to stab you in the chest with an Energy Sword or that glowing Promethean sword and you have to do a motion with the right or left stick (based on whether you are lefty or righty) and then your character punches them in the face, then you grab your knife/plasma dagger and kill the enemy and continue to fight. Also, you could press a certain button while struggling with the enemy and call an NPC ally to shoot the enemy off of you. This could mean a Marine or Elite, and with the Elite you could ask the Elite ally to physically pull the enemy off of you. This feature doesn't necessarily have to be in this exact method, but something like this would be cool.
What I want for Forge
Bigger Forge Maps
The game should have bigger maps for FORGING. The game could have some maps that aren't used in Multiplayer except in one or two gametypes, like a map that has two towers on opposite sides that is only used for CTF while there is another map that is completely unorganized and made for FFA Slayer. Also, there could be a Forge World similar to the one in Halo reach. Not the Forge Islands/Anvils map due to the fact that those islands were entirely flat and didn't seem like real landscape. This map could have a Beach with an ocean extending to the end of the map, and there could be islands near the outside so if someone wanted to make a beach landing map they could. There should also be a few mountains or at least large hills, like the mountain in the middle of the island on Halo Reach's Forge World. There could be a portion of the map that is a plain like the Forge Islands for people that want to make a trench-type battlefield. Then there should be a canyon like Blood Gulch in this map, and also a River that separates a plateau from a steady decline to the river. There could also be a tunnel from one side of a mountain or hill with wooden supports that look cosmetic. but if there is fighting and a bullet hits a support the cave collapses and kills anything inside. I would like this Forge world to be at least twice the size of the Halo Reach forge world. The Xbox One should have the graphical capability for this to work, so why not? It would make for more interesting gameplay.
No Budget Limit or Object Caps
I don't think Budget Limit or Object caps are particularly needed, because the only thing that they do is restrict the options of players. The reason for this could be a server limitation, but with the new console and hopefully new multiplayer engine (Frostbite please ) the limits could be extended to 1.5x, double or even triple the original $10000 amount. This could allow for a race track that spans the whole of my earlier mentioned Forge World, or simply a giant building that is a battlefield in itself. This also would let players spawn enough vehicles for NPCs to man, which I will explain later.
What I want for Firefight
I want Firefight back... Obviously
Firefight housed some incredible memories for my friends and me, and I want to relive them with a Promethean touch. Well, more than a touch. I want there to be an overall Firefight mode, but when in the Firefight menu you can choose between Promethean Firefight or Covenant Firefight, or have both. Also, instead of the Drop Pods and Snipers from Halo Reach, that options (hazards and stuff) could be Flood. Yes, there would be fighting between the Prometheans and Covenant but if they see you they will gang up on you. You could also use this infighting to your advantage because of a new Armour Ability that I would like to suggest, called "Veracity" or something because the Librarian showed Prometheans that humans weren't the enemy and changed them to non-hostile, only defended their world. You could change Didact Prometheans back into this form to help you in Firefight, or just play as Prometheans and Covenant as well. This new suggestion will be voiced later.
In all seriousness, this wouldn't be hard to do. Just make the old Firefight gametype into a new Multiplayer gametype. Firefight can still have its standalone menu that I mentioned in the before paragraph, but you could maybe have people forge their maps and then make spawn zones for the Covenant with moving doors. Or, as I saw in an earlier post, the players can choose where cosmetic things go so they add their own personal touch to a preset map, like a "Destroyed Warthog (3)" option. The (#) could be a dead body option, so that it looks like there has been fighting already and it would add a nice feel to Firefight that I haven't seen. Also, if you do completely outdo what I just typed and players Forge entire maps for Firefight to be played on, then I would greatly appreciate that. I could make a Lone Wolf type of map or have a center point that is easily defendable for 4-8 players (add more players please, bigger maps = more players+enemies) and the Covies come from one entire side and Prometheans from the other and the flood infection forms rain down from a crate that you can shoot the supports of.
What I want for Multiplayer
Better Assassinations w/ More Options
Assassinations were cool in Halo Reach and Halo 4, but I really think that feature could develop a little more. What I propose is that you can go up to someone and press the assassination button but based on the weapon you're holding or the faction you are, you do a different assassination. If you have an AR you can elbow your Spartan enemy in the back and then shoot them in the back of the head with your gun. You could also be able to wield a knife so you can do an old fashion assassination too. Also, maybe you could use Promethean teleporting so that they escape assassinations. That would be annoying but it would also make Prometheans a challenge to defeat in Multiplayer, which I think would balance out the fact that they have no vehicles... yet. A player could also be able to unlock different Assassinations based on how much they play. This is similar to the Loadouts with Halo 4 but these are purely cosmetic.
More Diverse Gameplay
One of these options would have to have the armours that give small bonuses, not enough to change gameplay but just to give people options. The Venator armour from halo 4 that you unlock when you assassinated however many people, this could have given you a 5% bonus in XP toward Assassination unlocks. Nothing in combat but for your character. There should be armor stations so that you could change 1 piece of armour on the go so you can adapt to the map that you are playing on. I know in Halo Reach I had certain styles of play for different maps, so they could change an armour piece to unlock more in that area because they get more XP or something.
Community Map Creation Events
I don't know about some people, but only being able to play on a few select maps really got me bored with Halo for a while. I mastered all the maps I liked and still mastered others that I didn't really care for. I would really like some diversity when it comes to map-making. In Halo Reach some maps were made by the community but this only happened once. I would like events like that regularly, every month or so. This would make it so you could never get bored with the maps and you always had new options to explore. This also means that newer players don't have a disadvantage when it comes to playing some maps because if they play on the newly released maps, the experienced and veteran players wouldn't know the exact layout of the map and every little detail.
I would like it to be extremely hard to get Player Ordinance so that it is an achievement and can be a turning point in a battle. I remember in Halo 4 playing on Exile when the other team had 2 experienced players that worked together and targeted all the noobs on my team and when they both had Incinerations cannons for Ordnance and they completely owned my team. This happened consecutively every 4 minutes of the match and it can be really annoying when they farm Ordnance points off of a select few players. This difficulty to get Ordnance can be counteracted by World Ordnance that spawns on a timer can be the same, with a few tweaks. That way the player ordnance can be as Overpowered as you want it to be but it is really hard to acquire, while any player can get World Ordnance and they spawn regularly so both teams have an equal chance to get them.
That's About It!
I hope that you, 343, take these suggestions into consideration in the very least. I would greatly appreciate it if these changes (at least some of them) were implemented into a game in one form or another.