IMO
All the things the previous poster mentioned plus:
Forge:
Magnets not properly lining up causing bumps in maps destroying flow.
No large scale forge maps.
Being able to put trait zones in blocks. e.g. way you could make a low gravity area inside of a couple of rooms without having to strategically place trait zones.
Killzones active while creating: I like to build my maps to the absolute maximum area I can, but keep dying from the kill areas while placing blocks around the outside, it would be better if it had a “warning outside of play area” popup or something that way you can still forge on the limits of the map area without dying repeatedly.
No flat sandbox type maps where you can build whatever you want from scratch without having to waste blocks on building a flat base
More lighting effects: (being able to place lights or modify lights on blocks to: change colour, pulse, dim, flash, flicker, etc.)
Custom games:
Not enough gametypes: (race, rocket race, invasion: (who cares if it’s only spartans vs spartans its the way the gametype works that makes it fun), territories: (I know dominion is supposedly to replace it but I don’t like the idea of fortifiable bases and reward systems that make it easier for the leading team to win), VIP, Insane: (the gametype which brought about many of the greatest halo reach minigametypes), Juggernaut, and Assault would be great for a start.
It would be nice to have some returning weapons from previous halo games for use in forge (bruteshot, Grenade launcher, missile pod, flamethrower, sentinel beam, focus rifle, needle rifle, Brute spiker, SMG. etc.)
Matchmaking:
Not enough gametypes: (see above)
Thruster pack too inconsistent: (the distance the thruster pack sends you is different on every playlist ranging from “pointless” to “out of there”)
Inability to play spartan ops you have offline.
I’ll probly have more later.