Ability to swim
Bosses after every other chapter.(fighting other Spartans somehow maybe)
Secret weapons
return of skulls/easter eggs
online campaign score
A difficulty setting higher than legendary
(multiplayer) Bots with difficulty setting.
Large spacecraft available
dual wielding
Unlockable armor/weapons/abilities/perks/anything else... through campaign achievements.
armor customizing in campaign (customizing armor isn't just aesthetics, they make you stronger/faster etc.(multiplayer too)
Better more comprehensive loadout options
skill and xp to return but easier to level up not like H3