Multiplayer, Pt. I:
This one would be the hardest of all to accomplish… Since they took so much different Routes Game after Game…
I would like them to lean heavily on Halo: Reach, but just from a Diversity Standpoint. Reach was the Golden Age of Diversity for Halo as a Franchise. You had dozens of Modes, Slayer, CtF, Oddball, King of the Hill, Invasion, Swat, Snipers, Stockpile, Headhunter… All the Submodes like Swatball, Swat KotH, One Bomb/Flag, Multibomb/flag, Bro Slayer, Superslayer, Hog Potato, etc. etc.
I think one of their biggest Mistakes with newer Halo’s were the lack of those at Release and long after shipping of these Games… I mean… I would guess, they saw the most played Game modes and thought “well that’s it, we just ship with the 5 most popular” without seeing that, while they might were the most popular, people could get really fast exhausted of playing the same over and over again and could literally jump in another Playlist for a Round or 2 and play something else in the past… This Option was taken from them in newer Games.
All in all I would like them to mash most of the Halo Plethora of Game Modes with just a few Exceptions.
*Throw Dominion and this KotH-esq Gamemode from Halo 5 out of the Window, they are bad Evolutions of past Modes
*Throw Warzone in the Trashcan, where it belongs… The First few Weeks it was amazing, but soon it got waaay to overloaded with heavy One-Hit Wonders and just was a Pain to Play… Additional to that, scrap like 70 Weapon Variants and 40 Vehicle Variants… If you want to make new Guns, make new Guns and don’t just give them a new Paint and call it a Day. Since we’re at the Scrap this and That Part, scrap ALL of the Armors from Halo 5, except Blue Team and Osiris (Both Campaign Variant Armors; especially Vales Armor got >insert bad bad word here< in MP), the Marks, and an Exception here and there… Later more…
(((*If Warzone for Money Purposes has to return, overhaul it’s Economy. Maybe, instead of having dozens of different Vehicles, Have One Base Vehicle and as your ReqCards, “Add-ons”.
So, in Case of the Mantis, you have the Baseline Thing for 2 or 3 ReqPoints, if you want to have a beefed up Version, you can unlock the Add-on (here: Shielding, AA-Rockets, EMP Rockets, Gauss-Triple-MG Turret, All-Seeking Rockets, Needle MG, Radar+, EMP Immunity + [just shorter EMP Status], and so on) and for every Add-on you attach, it gets more expensive. Make it, that you can have already build up permanent Slots, that you don’t have to modify your Req over and over again. In case of Weapon, Vehicle and other Req Economy: Don’t let the Players bring their whole Arsenal… Let them choose maybe 40 Reqs (I dunno which Number to choose since it’s all hypothetical) they can bring (in addition to the permanent Ones) or maybe give them a Set Amount of ReqPoints to spend for a Game to prevent ReqSpamming. Additional to that have a rotational Warzone Playlist (Normal/Speed/is there something else? 

), that Folks, who love the Standard Mode they got to know in 5, still can play that.))) → In the End I don’t want Warzone, but with a few tweaks here and there I could live with it… 
*Bring back Invasion, but on a larger Scale. It is hands down one of the best Casual Modes (if not the best Casual Mode) Halo ever created…
Leave it how it is, Spartan vs. Elites with Elites having different Attributes, than Spartans (more Health, more Speed/Jump Hight, Bigger, but not as Halo Reach Bigger, the Normal bigger, I would like them to have the Halo 2: Anniversary Models, they captured the Grace and Honor of this Species the Best…)
Maybe extend the Rounds within the Rounds, by that I mean having like 3 or 4 Phases before stealing the Core…
Let it have JiP because Rounds are getting long this way and so People can come and leave without getting their Team in Trouble.
Maybe have different Phases… Not only capture this and that (Breakpoint did make a start in that direction by making the 2nd Phase a Plant-the-Bomb Stage)
Invasion should be the only Mode with Premade Loudouts, I think they nailed it there from a fair Options Standpoint…
The Expanding in Gameplay should be just like it is now… You get a Phase further, more Options in Form of Power Weapons and Vehicles (and Loadouts) spawn on the Map
*Bring back every Halo Reach Mode (it just saves me some Time writing xD)
*Bring back Regicide, Team Regicide, add expected Modes just as Infection, Griffball, Action Sack Modes
*Customization should also heavily lean on Reach, take it as a Baseline, expand on it with this Options:
Undersuit (not the Color, it should be Black-ish, I mean the Pattern, like the Ones from the H5 Campaign, they were unique and not standardized), Visor/HUD Style (also, H5 Campaign) & In Game “Companion” Voice Actor (also from Halo 5, I mean the Exuberant Witness, Buck or Standard Voice for In Game MP Lines)
Add all the Elite Armor ever seen till to Date (except Arbiter), that we have like 20 Options or so:
The Ones from Reach, but more Halo 2: Anniversariesed + Honor Guard, Honor Guard Councillor, Councilors, the Shipmaster from Halo Wars 2 and the Ones with the Red Energy Sword from Halo Wars 2 also, etc. etc.
*I would like to have the Option (and I’m really alone with that I found out) to turn on Dual Wielding… It shouldn’t be mandatory, I just would like to have a little Switch somewhere saying “Turn Dual Wield On: [Yes]/No”, it never has to be in Matchmaking MP, I just want it for Fun Purposes and Nostalgia in Campaign and Custom Games, even if Dual Wield Weapons like Dual Needlers would be hilariously Overpowered, it’s just for a few Fun Games with Friends after all…
*Weapon Models should mainly be taken from Halo 2: Anniversary (+ Artstyle), since that was the Most Beautiful One, in my Opinion. Multiplayer Maps should also again Focus on not just play good but also look good instead of blend… A few Showcases would be All of Halo 2: Anniversary Maps, Halo Reach: Halo CEA Map Pack (Solitary, High Noon, Battle Creek, etc.), Halo 4’s Pitfall (Pit Remake), Solace (Halo 4), the green Shiney BtB Map on some Moon (I think), Avalanche (Halo 3), Spire (Reach), Perdition (Halo 4), Waterworks (Halo 2: OG), Attack on Sanctum (Guardians), Longshore/Offshore? (Halo 3), in General Additional Maps that got released in Map Packs or Updates, they seem to recieve more Love from Developers…
Only a few few Forge Maps, if the quality doesn’t crank up like 5 Times… Many many play very nicely, but they look just… °~°
*Ranking Vise should be different Systems at Work:
- a Credit Based System from Reach, where you can purchase Armor from (if u hit the required Rank for Purchase) and Rank up your Global Rang (1-20.000.000cR; Recruit to Inheritor, that was the right Mix of Playtime Investment to not be to short nor to long)
- 1 to 50 from Halo 3 for Ranked Playlists
- Playlist Ranks just like Halo 3 did introduce in it’s later Years (a white Military Grade Symbol in every Playlist going up the more you play said Playlist)
- A Tournament Rank System for a Monthly Resetting Tournament Playlist (Bronze I to Champion [200-1], just like “Guardians” introduced and just for 1 or 2 monthly Changing Tournament Playlists)