Disclaimer: these are my opinions
Loadouts should not be as extensive/over-complicated as they are in Halo 4. Make it where your primary weapon choices are: AR, BR, or DMR (no reason, canonically or otherwise, why S-IVs are issued Covenant/Promethean weapons). Same goes for sidearms and grenades, so I think Magnum and Frags should be default and unchangeable (this gets rid of the problems with N00B combos, PP/Plasma grenades, and Boltshot at spawn). Resupply is now, or rather once again, a default characteristic (no reason to over-complicate the elegant “golden triangle” dynamic of firearm-grenade-melee that has been around since CE). That said, no support upgrades/tactical packages or AAs at spawn. Rather than having Sprint as an innate ability of all players, increase base player speed and allow the choice between Sprint and Evade as such (choose between increased mobility or maneuverability).
I feel that the Covenant arsenal should once again include the Needle Rifle (balanced to be the counterpart to the DMR, as the Storm Rilfe is to the AR and the Carbine is to the BR). Brute weapons and grenades should also return, with the Brute Shot (replacing Concussion Rifle), Mauler (replacing Boltshot), Spiker Rifle (heftier Spiker with Retro Lancer-like design), and Spike/Incendiary grenades. Brute vehicles, mainly the Brute Chopper, should also return.
The Promethean arsenal should receive a damage boost, making them more effective versions of their UNSC/Covenant counterparts. The Boltshot, however, should stay out of multiplayer.
Armor Abilities and weapons, not just a few power weapons, should be placed on the map. This puts more emphasis on map control and should remain a recognized staple of Halo.
Ordinance should only be able to drop Armor Abilities. That said, a few AAs that I think should return (or be made) are: Bubble Shield, Regen Field, Hologram, Active Camo (but with changes to promote movement rather than camping), Gravity Lift (substitute for Jetpack in multiplayer), and the EMP Mine (releases EMP to drop shields and stop vehicles, but doesn’t kill). The thing that made Equipment work seems to be that they affected the map, rather than affecting how the players worked. i think 343i needs to keep that in mind when designing/balancing AAs in the future.

