Campaign:
Matchmaking feature should be added like it was in Reach. It was a small touch, but it encouraged players to play the campaign if they had a challenge to complete and didn’t want to jump in by themselves. An option for difficulty should be a voting option at the start of the game for all the players in the lobby. They could search by being able to choose any of the campaign levels, like in Spartan Ops.
Hide skulls before they can be used.
Campaign scoring option.
Less button pushing, if you know what I mean. (Halo 4)
If you are playing 4-player co-op, let the party leader be Master Chief and the others be their custom spartan. Reflect this in cut scenes when and if necessary.
War Games:
Certain playlists should have objective and slayer gametypes, like BTB and MLG. Keep playlists consolidated for better population.
Bring back double exp weekends.
Ammo should be available around the maps for primary weapons. Maybe just an ammo crate for whatever primary weapon you are carrying. Similar to Spartan Ops.
Vehicles should be buffed because too many people on maps have power weapons with ordinance drops, or improve ordance drops. Have them happen every 8 kills, or 5 if you get them more frequently. Let us customize our ordance drops to a nade choice, an armor ability choice and a speed boost/damage boost choice.
Put power weapons around the map only.
You should rank up faster if you win more, maybe offer a flat 1000 exp if you win, and you lose 1000 exp if you quit. Either way, encourage players to win and not quit.
Make it so that commendations can be adjusted by 343 if completing them is too easy, or too difficult.
Bring back de-scope when taking fire.
Bring back proper medal chest and all emblems from previous Halo games.
Bring back armor effects, and my katana, and let me use that katana in assassination animations.
Join in Progress needs to be fixed entirely. Don’t start games without a full lobby. Just give people the option to turn Join in Progress off. Everybody is happy.
Allow us to view party details so we don’t join a queue just to find out your friends’ game just started.
Dedicated servers need to be well maintained.
Better Ranking system, experience and skill based.
Spartan Ops:
Difficulty should be a voting option at the start of the game for all players in the lobby.
Give us firefight or something new to replace Spartan Ops.
Theater:
This feature seems to suffer with every new Halo game. Theater should be perfected and left alone. You should have direct access to your recent films from the Theater lobby. Allow us to watch any and all recent game play, in campaign, Spartan Ops, customs and matchmaking. Perhaps have a quick option at the end of any game that reminds you to save a film would be nice. At least 4 players should be able to watch a film and/or a film clip together.
Integrate the DVR feature where we can pull clips back and forth directly from our fileshare.
Bring back the Community Spotlight and 343 Favorites. Allow us to access files from Waypoint and in game. Have films rendered automatically as part of the upload process. Sharing files made in Theater could be amazing.
Get File Share at least on par with Bungie’s version.
Other:
Better Maps - Maybe a Classic Map Pack, or one classic map in each DLC pack
More Custom Game/Forge options
Better Menu Navigation
Exclusive armor, Can I haz reconz?
Improved and customizable service records. That would be amazing, especially if I can include legacy stats from previous Halo titles.
Better Marketing! Minimize preorder bonuses, don’t exclude parts of the world from DLC and if you buy a Halo 5 console, include all the limited edition exclusives with it.
Each Halo game following this should vary on maps, weapons, armor and then only add features that won’t completely break core Halo.