1: Elites, designed in a way that they don't need to be locked out of certain gametypes (I.E. don't do what Reach did). I suggest simply shrinking their multiplayer model to be the same size as a Spartans while standing upright. See here
2: A non-RNG unlock system for cosmetics. Any and all microtransactions must be directly purchased, they are unavoidable so I won't even ask for them not to be in the game. However there should be no microtransactions for at the very least base armor sets. And there should be a satisfying amount of options for attachments, and coatings unlockable through gameplay.
2.5: On the topic of unlock systems for non-microtransaction items, make it challenge based. Commendation unlocks in Halo 4 were easily the best system, but you could go even further by changing Commendation tiers unlock for completing feats, and thematically tying that commendation to what it unlocks. Shotguns are an easy one to explain so here goes; Kelly's primary weapon is a Shotgun, so you could tie that to Shotgun Commendations by having her armor and colors unlock through them. Shotgun commendations would have objectives like "Get a Killtacular with a Shotgun", "Get 100 Kills with a Shotgun", "Get a Buck Wild medal" etc, with each objective completed raising the Shotgun's Commendation tier. There would be 10 objectives, so a Commendation's max tier is 10. For each tier you unlock one of the following items: Hermes Helmet, Arms, Legs, Chest, Breacher wrist attachment, Kelly's colors as an armor coating, and vehicle coating, Oathsworn Bulldog coating, Shotgun-themed emblem, and nameplate. That's 10 rewards, for 10 commendation tiers that all are tied thematically to using a Shotgun. You can do the same for Snipers and Linda/Jun, Spartan Lasers and Gungnir armor, Energy Swords and Helioskrill/Arbiter themed gear etc.
3: Diversification between UNSC and non-unsc weaponry. The Beam Rifle is boring, because it's just a slightly better Sniper Rifle, the Focus Rifle may have been pretty lackluster as an equivalent, but at least it offered a completely unique gameplay experience so go for stuff like that. UNSC weapons should be the basics, they do exactly what you expect them to do. Covenant weaponry should always be different, with unique gameplay elements like stunning effects, overcharges, super combines, and sticky explosions, I've seen a fantastic recommendation for Fuel Rod cannons that they should act more like mortars that create pools of radiation for a period of time than just being the Covenant Rocket Launchers. Brute weapons should be powerful, but clumsy and somewhat unpredictable, requiring you to fight a bouncy recoil with each shot, or a Goldeneye-eque sway while zoomed in with a bolt-action 1 hit kill Brute sniper. Forerunner weapons should be raw powerhouses to the point that equivalents to standard weapons should feel best-in-class like mini-power weapons, but are rare to find and with limited ammo supplies (Like 1 magazine limited).
Bonus 4: Include some maps that are geometrically designed (I.E. Designed with large simple shapes and angles) as opposed to all of them being designed to feel like a real place. Great maps like Lockout, Damnation, Sanctuary, Narrows, Haven, Midship etc were all based around very simple shapes and designs that gave multiplayer an arena feel. In Halo 5 that felt pretty well reserved for Breakout which was unfortunate to say the least. Even their remake of Midship was redesigned in a way that overcomplicated it, at least we got Coliseum. Not to say more 'realistic' map design is unwelcome, other amazing maps like Ivory Tower, Ghost Town, and Zanzibar come to mind, but I'd like to see more of it in Infinite.