I’d take Halo CE as the base of my Halo 5.
I’d add on top of that:
-Multigravitational maps, gravity can be in several directions on one map
-Zero G combat, non gravity areas or maps
-Utility based weapons having other roles and functions than normal weapons
-Health would generate with health packs or when close to a team mate.
-Controllable elements on the maps akin to those found in H2A, but bigger in scope at some places
-Hijacking with the added feature if using certain weapons, such as the sword or shotgun, for added speed and efficiency
-AI which learns and improves.
-Jump crouching, crouching and then jumping gives you extra height.
-VISR, thermal and nightvision modes instead of flashlight.
-Difficulty based on AI intelligence rather than brute force increasind and you as a player becoming a crystal glass.
Campaign:
-Branching missions and side objectives changing how things play in later missions, not branched story though. Chosing to destroy either a Wraiyh facility or Banshee hangar would remove or reduce the amount of wraiths or banshees in certain levels.
-Timed events changing how a mission plays depending on how fast you can do something. Example being a door that will close in 1 minute, not getting through the door in that time wouldn’t mean mission failure, but you need to take a longer different route.
-Your multiplayer spartan present in the campaign in some way.
-Multiplayer objectives reworked as mission objectives, CTF could be remade in such a way where you defend a specific object from capture and try to capture a vital object or several from the enemy, and so forth
-Players can join the enemy side and select different units to play as against the player.
Firefight:
-New Mombasa size maps.
-Dynamic overseer AI spawning enemies depending on what the players do and how they behave.
-Random events and objectives which provide different rewards when completed.
-Flood, “Covenant”, Humans and Forerunner sides available to fight against or with against the other non allies, or a big FFA.
-Each race having it’s own overseer reacting differently to the same events.
Multiplayer
-Map vetoing outside match making. You select one or two maps from each playlist map rotation you do not wish to play on and when you jump into matchmaking you’re assigned a map and gametype with no voting or vetoing.
-Random events alerted about long beforw they happen, and what will happen.
-Que notification when someone ques to join you.
Forge:
-Version of Project Spark as map editor and scripting systen. Customized for my version of Halo 5 of course.
-Able to forge for all game modes except campaign.
-Forge AI, an AI that learns how maps are built abd can navigate them, be it multiplayer bots and regylar NPC units. Or can be scripted by the player for apecific behaviours, or a mixture, say how it behaves and learns after that.
New mode: Scenario
-Short-long single mission
-Functions like a normal campaign mission but isn’t part of a campaign.
Theatre:
-Can view from all game modes.
-Ability to lock camera onto objects present or soon to exist such as unfired rockets which will be fired in 10 seconds.
-Chain together lock ons so that you can jump from rocket to rocket to grenade fast and efficiently
-Filters working so that you can choose which object gets filtered and not. For instance, black and white filter but players retain their color.
Assets:
-More vehicles, new and old.
-Different base armors all with addons you can attach individually
-More gametypes, new and old.
-Saved film clips should be usable as main menu background, mode specific clips used for mode specific menus.
That’s what I can think of.