Speaking of fire grenades, a system that lets us choose which grenades we carry with us (Perhaps a system similar to how we swap weapons) is a must if we're going to have three or more grenades.

: 2) Heads-up Display. The HUD needs to have a health bar, a secondary weapon
: indicator, and a compass. Because some information I just need to know.

Don't forget the sprint-meter!
And get rid of the clutter! I don't care if you do hurt some graphic designer's delicate feelings.

: 3) Controls. The number of layouts needs to expand beyond the half-dozen or
: so we typically get. At the very least we need an additional layout
: similar to the default layouts from Halo 1 & 2, i.e., something like
: this:

No.
Absolutely not.
Melee BELONGS on a bumper. Would you put "Fire weapon" or "Throw Grenade" on the B button?

: 2) Level design. You could have made a drinking game out of how often the
: phrase "open world" was mentioned out of E3 (in fact, if you
: did, there would probably be no survivors).

Challenge accepted.

: 5) Enemies. Bring back the Brutes, the Drones, and Engineers. Unless they can
: be reworked to be an enemy that is interesting and fun to fight, then
: scrap the Prometheans; say they were all wiped out when Requiem was
: destroyed.

Dayorder: unbeing Prometheans fullwise. Recdep rectify misprint.

: 8) Difficulty tuning. Ever since Halo 2, it's been the trend to make enemies
: as overwhelmingly powerful as possible on Legendary. However, it's a
: design decision I disagree with. It considerably reduces the amount of
: viable tactics available to the player, essentially shoehorning them into
: a very specific play style — usually one that discourages aggressive
: movement and that’s heavily cover-based —, one that if deviated from will
: result in repeated death due to the extreme fragility of the player. Most
: of the challenge of facing an individual enemy should come from things
: like quality AI rather than brute force (no pun intended). As the saying
: goes, fight smarter, not harder. That should apply not just to the player
: but to the NPCs as well. If the enemies are sufficiently intelligent then
: they wouldn't need to inflict absurd amounts of damage; in fact, if they
: were really smart and inflicted as much damage as they did in Halo 2 or 3,
: it would result in the game being rather cheap on higher difficulties
: (refer again to Reach). There are plenty of other factors that go into a
: game’s challenge, including but not limited to: • Overall resilience of
: the enemy (i.e., shield and health levels).
: • The performance of the enemy AI.
: • The presence, number, and general effectiveness of friendly AI.
: • Overall effectiveness/performance of the player’s weapons (damage,
: accuracy, etc.).
: • Whether or not the player has regenerating health.
: • Level design, which affects the flow of combat.
: • Encounter design, i.e., number, placement, and rank distribution of the
: enemies, and presence/absence of multi-sided battles.
: • Availability of ammo, weapons, and other supplies.
: • The weapons and other gear the enemies are equipped with.

Honestly, I think that the enemies in Reach are a pretty good place to start if we're making The Greatest Halo Game Of All Time. I loved the Elites because they were mobile, smart, and dangerous.
So, take the Reach enemies and incorporate these changes:

Elites:
• Modify the melee. The punch is powerful, but no longer an instant-kill. At the very least, it breaks your shields.
• The kick does less damage, but pushes the player back a few meters.
• No more precognition. If you're halfway across the map with a sniper rifle and you are settling the crosshairs on an Elite's head, it shouldn't immediately sense your presence.

Grunts
• Bring back the T-42 plasma cannon from Halo 2.
• Keep them silly and speaking English. They aren't just cannon fodder, they're comic relief.

Brutes
• I think that a compromise between the Halo 3 Brutes and the Reach Brutes would be great.
• Reach-style Brutes should be uncoordinated and operate somewhat independently of each other.
• Halo 3 style Brutes should emphasize and improve the Brute Pack. Fire and maneuver, coordinated grenade volleys, the whole shebang.

Skirmishers
• Pretty please?

Drones
• The only improvement from Reach that I can think of is to make them more prone to landing.

: MULTIPLAYER

: 1) Get back to basics. No more perks, no more weapon unlocks, no more custom
: loadouts, no more care packages personal ordnance. That belongs in games
: like COD, not Halo. Put everybody back on a level playing field, which
: means everybody starts with the same gear and same abilities. Armor
: abilities should, as a rule, be items you have to pick up off the
: battlefield. If they exist, loadouts should not be the norm (restrict them
: to gametypes like Invasion), and when they are enabled, they should be
: presets.

I don't care in the slightest about Multiplayer, but some people are going to want to be able to edit loadouts for custom games. If you mention this later, apologies.

: OTHER STUFF

: 1) Art Design. Ditch those freakish mutants that the art team thought were
: good designs for the Jackals and Grunts.

Interesting.
You stated elsewhere that animals with such bizarrely different physiology wouldn't even be in the same genus.
How radically can congenital defects change a person's skeletal structure?

: I don't mind new armor designs to
: mark a new faction, but to radically change alien physiologies in a manner
: that defies all sense and reason just shatters immersion. It's already too
: late to salvage the changes to the Dawn and the Chief's armor,

The Hell it is.

Halo 5: Open with the Master Chief clad in armor that is a direct interpretation of the Mark VI. Never acknowledge the abomination from Halo 4 ever again.
Problem solved.

: 2) User Interface. Get rid of that horrid UI from Halo 4 and just reuse the
: one from Reach.

Yes, PLEASE!

: 5) And for the love of God, have an actual, real, full-color, full-length,
: made-of-paper instruction manual. These former mainstays are starting to
: become an endangered species, and that makes me a sad panda.

Earth first! We'll log the other planets later!