A few more ideas for my wishlist.
Campaign: Weapon customization.
Halo 5 showed you guys know how to make lots of cool and fun variants to every type of weapon.
Complete the campaign once. The Campaign Weapon Customization menu unlocks allowing you to swap in/out different variants of weapons already placed in the levels. Example: replace a standard UNSC raingun with one that charges faster, doesn't auto-release, has secondary explosives. etc.. I think you guys understand what I mean.
Just to make it more frustrating, unlock the Campaign Weapon Customization menu only for the difficulty you completed. Meaning you have to complete it with normal weapons at least four times.
FireFight levels (lesser extent multiplayer levels):
Playing FireFight so much I thought of an interesting idea. Have the level physically change after every Round. For realism, what type of level could physically change after every round. At the moment I could only think of Forerunner and Flood style architecture reconfiguring itself after every Round (not Wave). In this instance the Flood architecture doesn't need to feature Flood enemies, it's just about the level itself and why it moves.
New FireFight Type:
It would be cool to have a FireFight Ops version. This plays like regular FireFight (not Spartan Ops), but after every Round or perhaps Set a new random goal or type is chosen, meaning the first Round you defeat enemies, but potentially the next Round is Generator Defense, or perhaps, a FireFight version of Invasion/Cature the Flag, Assault, Territories, and new types of goals thrown into the randomizer. Basically, this is just FireFight with no story like Spartan Ops and a gametype randomizer after every Round. Need to teach Covenant to carry and plant bombs!
New FireFight Type: Limited Flood style FireFight.
Flood Hive style level with the reconfiguring mechanic mentioned above. You are fighting regular enemies within the Hive. When the Boss Round starts at the end of the Set, you have a limited amount of time to get to the safe room. Then masses of Flood spore emerge and take over the bodies of the slain. If you don't make it to the safe room in time you lose a life. Within the safe room, which only opens at this time, are various goodies, maybe even a vehicle, and the important back door. This back door allows you to get back into the level fairly without being mobbed by dozens of Flood types at the front door. After this end of Set Boss Round, you go back to fighting regular enemies. Make it to the next Boss round an our little Floody friends emerge again.
EDIT 2019 Aug 05:
FireFight 'music' idea.
In FireFight on the Boss Round we've had drums signalling danger ahead. One thing I'd like to see or hear that is, would be the Boss Round and maybe the preceding Round featuring 'Uphill Both Ways' -original ODST track, and the remixed track from the Halo Reach multiplayer reveal trailer. There could be an option to turn it on and off too.
EDIT 2019 Aug 12:
Would like a FireFight level that takes place on a Mammoth moving through a landscape. The Mammoth should be more stable as well as other vehicle and NPC interactions with it. Also I loved that Spartan Ops level where you need to board and destroy the Lich. Boarding and destroying Lich's could be a type of Boss Round so the Mammoth can continue through the landscape.