I don’t really know if 343i actually reads their users threads, but if they’re as good as they sound, they should 
Anyways lets begin… (Just to let you know, I’ve only played Halo Wars, Halo 3 ODST, and Reach.)
Armory Ideas:
-Elite Armory if they still have elites
-Every helmet from the past games, plus more.
-Instead of cR, bring back EARNING the helmets.
-If you like cR, maybe a combo. Either you must pay this more or do this. Or some things are priced, some things are ment to be earned.
-Much more visor colors
-More Primary and Secondary colors
-More than just 2 attachments per helmet
-More of everything, as in Helmets, Chests, Shoulders. EVERYTHING, expecially armor effects.
-Maybe an option to wear your helmet or not like in ODST
-If so, customizing your guy/girl’s face.
(These are just off the top of my head, I’ll add more if I think of anything.
Game Ideas:
-Instead of small little maps, maybe a bit, or a lot bigger?
-Weapon choices maybe? Thats one thing I like about CoD more. Infact, thats the only thing. But still. I liked Classes and stuff. I don’t know if Halo has always been picking up weapons, if so, maybe it’d be a bad transaction.
-NO ARMOR LOCK
-More armor things (like jetpack or sprint)
-Maybe showing the loction where the murder killed you, or kill cams, or something.
Forge Ideas:
-More of the vehicles like Scarab or I heard something about an Elephant, etc.
-Make it so its not all just grass and water. Make like islands and one be grassy, one desserty, maybe one foresty, etc.
-Idea chart: for jotting down ideas to look at later
-Either no price limit, or a bigger limit.
-Changing armor in game, because doesn’t really matter if your just in forge
-A guide on how to do stuff would be nice, I never found one people just told me. (Same with Custom games, how to make them)
I don’t know what else to put. If I get more ideas I’ll just add them on here, but til then, hope you enjoy.