Let's start with Campaign
Campaign Ideas :
New Skulls
Mythic Difficulty(The highest was legendary now add another one mythic)
Audio logs(Like in ODST)
Map
Easter Eggs
Bigger & longer levels
More levels
Co-op
4 player splitscreen co-op
Smart A.I
More Flood Creatures(Juggernaut,Infection Form,Carrier Form,Infected Marine,Infected Forerunner,Infected Hunters,Infected Elite,Flood Swarm,Thrasher form from halo wars,Bomber forms form halo wars,Infected Spartan)
Flood & Forerunner Buildings
Boss Fights
Firefight Ideas:
More enemies like the Flood & The Guardians(Sentinels & Monitors)
Forge Firefight map
Smart A.I
Multiplayer Ideas:
A Mixed Reach/3 Ranking System (You get EXP for winning games and you rank up but with no skill level so you level up a bit faster and you get credits by playing a match credits are only for customization this does not affect your rank while exp is for level up )
More than 50 Ranks
New Gametypes
More players like 32 (16 vs 16 Match)
Tournaments
Search Custom Games
Matchmaking
If you don't have new maps you would still be able to play on the Matchmaking Game you chose but only at the older maps
Vote for a map
More Balanced Matchmaking it will only find peoples on same a little bit lower or a little bit higher but not extremely low for your rank or extreme high for your rank
Splitscreen
System-link
Be able to play as a Brute
Armor Abilities(Regeneration,Invisibility,Sprint,Hologram,Evade,Radar Jammer & Teleport)
Equipments(Bubble shield,Deployable cover,Flare,Portable Gravity lift,Power Drain & Tripmine)
Select more than 1 Armor ability so you can mix it
Carry more than 1 equipment switch to a Equipment by pressing a button or button combination
Bots
Wager(Bet your Credits and if you win you will win all the credits sometimes objectives Gametypes or Slayer Gametypes and Last man Standing Gametypes if you win in these matches you will only win Credits for armor Customization you won't win any exp only in Matchmaking)
Forge Ideas :
Civilian Buidings
Civilian Vehicles
Dead Bodies
Statue's
Traps
More Light effects
Pelicans,Phantom,Scarab and other big vehicles
Change the water level
Lava
Rain,Snow,Lightning(Weather Effects)
Other Ideas:
Forerunner Weapons
Grenades(Frag,Plasma,Spike,Flame,EMP & some Forerunner Grenades)
Big Vehicles like Scarab,Phantom,Pelican and some new ones Driveable
All Weapons from Previous Halo
Theater
Record voice in Theater
File Share
343 Favorites(Something like the Bungie favorites from Halo 3 and Reac)
Dual Wield
Carry more then 2 weapons 3 Weapons Would be good
More Melee Weapons(Katana,Energy Sword,Gravity Hammer,Knife(Name more if you want...) )
Invite Friends
Elites also alot of customization
Brute Playable & Customization
(Edit)
Gametypes Ideas:
Last Man Standing(Everyone have 1 life and the one that survived win the round you win points by making kills and survive)
Team Last man Standing(Everyone have 1 life kill all opponents and if your team survives your team wins the round)
Slayer
Team Slayer
V.I.P
Assault
Territories
CTF
Juggernaut
King of The Hill
Race
Oddball
Escort(Escort the Leader of your team to a Base to win round Leader have 1 life so if he got killed you will lose the round)
Destroy The Building(Every Team has a building to protect the team that bombed or hacked a important building wins)
Dual Tag (2 Players got 1 life They need to finish objectives and kill opponents to get points if the 2 players got killed it switch to other 2 player the 2 players with most points win)
Invasion
Headhunter
Stockpile
Warfare(One Team must conquer Frontlines by doing objectives and another team must protect the frontlines and one team are the Rebels that Tries to get points by killing their enemies if a player from another team get killed by a rebel it can't spawn anymore Rebels got 1 life)