Attachable scopes, silencers, extended mags, chainsaws all added to your gun during the heat of battle to suit any unique situation.
FOR EXAMPLE. You find yourself at the bottom of an unclimbable rock face so you swap out your shotgun under attachment for a grappling hook, rather than find some other way around. Think of the possibilities.
New villains that don’t look like Bionicle monsters
FOR EXAMPLE. Leprechauns, Vikings, Giant Beavers
-Yoink!- naked Cortana replacement
The ability to kill your npc friends thus altering the plot of the game.
I want a unicorn so I can ride a rainbow into battle and be covered in glitter and happiness.
Also,
It would be cool to modify vehicles for multiplayer, warthog races or something where you can modify the guns, color, put some sort of boosts so not only do you have the option to customize your armor but also your ride.
Better female spartan vocals, she sounds like a butch chick.