Though perhaps a little obvious, I feel that it'd make sense for Chief's tilt attacks to utilise the Type-1 Energy Weapon, typically known as the Plasma Sword. It goes without saying that the Plasma Sword is an icon of the Halo franchise and singlehandedly probably one of, if not the most, iconic weapons from the entire series... so if it wasn't included somewhere around here, I think people would be disappointed. As for how it'd affect his moveset, I think it'd predominantly act as a tool to aid in zoning and edgeguarding - keeping opponents at a solid distance so he can lay into them with his more potent ranged attacks. However, to prevent spam, each of these attacks are slightly offset by above-average lag both before and after using them, due to the activation and deactivation of the blade itself.
All of the Plasma Sword attacks are unique in that they have a pretty unique visual effect to them - specifically, when swinging, the sword leaves a momentary glowing trail in its pathway. Not only this, but the iconic sound of its activation also occurs whenever you start up any one of these attacks (more akin to Halo 2's, given it's generally preferred) - and the sword is deactivated after the attacks end. Though, beyond those little additions, they're pretty ordinary in function.
Side Tilt: Drawing the Plasma Sword, Master Chief plants one foot in front of him and slashes it horizontally in front of him, in reference to one of his melee animations from the Halo series when using the weapon (if a little more pronounced.) The attack can be angled slightly upward or downward when used, meaning it's got a little bit more versatility, but it otherwise lacks in vertical coverage despite its decent horizontal reach.
Up Tilt: Master Chief performs an overhead slash downward from above him, being a more pronounced form of his other typical melee animation from the Halo series (which, being first-person, doesn't have as much range.) It's good for overhead and forward coverage and has decent range, making it a good anti-air, but its lack of coverage behind him can mean it's sometimes a little risky to throw out against opponents directly overhead.
Down Tilt: Master Chief draws the Plasma Sword and thrusts it in a forward-down angle in front of him whilst stepping forward, before suddenly lifting it up toward the end (with a sweet-spot) in reference to the Lunge attacks from the Halo series. As a down tilt, it has pretty surprising range... albeit middling damage and knockback, though this is offset by being a fantastic ledgeguarding tool for opponents close to the ledge.