Just my opinion, feel free to add on.
-Various computers (as seen in H3’s Forge, Zanzibar/ H2A’s forge scripted computer screen, Halo CE forerunner holo panels)
-More air vehicles (such as Pelicans, and I don’t mean the Banshee prefabs either)
-AI (not going to happen, but imagine playing infection and having a guard of two Wardens or Hunters, or role playing attacks on Covenant bases, or even remaking campaign missions from past games)
-Classic Gungoose (muh SMG gungoose)
-Roads (muh roads)
-Water FX (I mean real effects, not just noises)
-Flood pieces (prefabs of this are great, but some things just can’t be replicated by players)
-More holograms (such as Halo Rings, the Ark, covenant and human ships, and AI holograms such as Cortana, BB, Kalmyia, ect.)
-More skyboxes (self explanatory; I just mainly want a skybox win a huge space battle going on in the background to justify making a dog fight map in space)
-More Pelican pieces (such as wings)
-More corpses (Spartan bodies, marine bodies, broken Forerunner machines like Sentinels or Enforcer Sentinels)
- More props in general (such as those Sentinels I mentioned; making them AIs would get bonus points)
-Forge World (FOOOOORGE WOOOOORLD! Woooooo!)
-Flame weapons (this is a general gameplay thing, but would be neat to have either way… Firebombs were fun. Flame throwers were good for playing Burn the Bridge)
-More destructible pieces (see- Battlefield)
-UNSC air (for the entire game)
This list is getting long, so I’ll wrap it up…
-Destroyed vehicles (I’ll admit, I don’t think 343 can beat the community with Pelican crash pieces)
-Monuments (such as the Noble Team monument that came with Legendary Reach, or Master Chief monument as seen in Hunt the Truth)
-Wildlife (this might fall under AI again, but having fish, bats, rats, or other animals would be neat. Those things from Reach as a pet would be fun, the Star Wars remake would come quickly)