Gameplay wise? Get rid of Smart-Scope. It’s actually not that useful in-game, and it just pisses off the old-timers. Spartan charge and ground pound need to go - perhaps they could return in the form of some kind of revised type of armour ability? I like that idea. These armor abilities, however, are particularly lethal and work more like power weapons - not everyone on a team will have them at once. Things in bold are new ideas I just kinda came up with on the spot.
Armour abilities - ground pound (larger area of effect), spartan charge (sends targets that are hit flying a great distance back in the air and stuns), jetpack (extended fuel over previous titles), bubble shield (repulses enemies, allows allies to shoot through), Electromagnetic Pulse Emitter (basically blasts everything in a certain radius with the same effect as an overcharged plasma pistol, including the user of the ability, leaving vehicles stunned and all Spartans shield-less), active camo (causes the user to become completely invisible even when moving quickly, except if really close and the shimmer can be seen. If they sprint, jump, melee, or fire,enemy VISR temporarily outlines them in red if active. Active-camo is automatically de-activated if the user’s shields are broken, and shield flaring can be seen with camo engaged if the target is hit), Smart-Linked Auto-Targeting Module (Soldier 76’s tactical visor from Overwatch for a limited period, focuses on the centre-of-mass, not the head. Canonically a module containing a very expensive, specialised dumb-AI which takes over the position of the user’s weapon to focus on high-value targets with expert precision).
Spartan abilities - sprint, thruster pack, slide, hijack, clamber, VISR (highlights the environment and detects things of interest - highlights fast-moving active active camo users but the user gets visual interference when hit by plasma or EMP, forcing them to de-activate it).
Y’know something else that would be really badass in Warzone? Off-map call in strikes. For REQ energy, you can but things like AI ODST squads which land and murder enemies in an area, guided Lancet micro-missile drone strikes, Mini-MAC blasts, tankbuster strafing runs. I know it seems pretty COD-ified, but it’d be certainly badass with a Halo spin and only in Warzone.