We need to be able to add AI in forge mode and put custom Firefight maps in our file share. This absence in Halo 4 is my biggest complaint about the game since we've been asking for it forever. Other than that, I'd love it if the game allowed you to customize flood starting weapons and be able to spawn players with no weapons. I'd also like a campaign that's a bit more open. Halo 4 was REALLY linear and I got bored easily. Instead of forging pre-existing maps, we should have a flat map specifically for forging (War Games and Firefight) that we are able to terraform. Don't tell me it's not possible: Farcry 3 did it and I think it would be mind-blowing if Halo had this along with its already remarkable gametype customization. Also, being able to change the biome would help.
Recap:
-Ability to forge campaign/firefight maps.
-Flood being able to carry weapons.
-Gametypes where players spawn with no weapons.
-Less linear campaign.
-A better forge mode with terraforming.
-Changing weather in forge.
And PLEASE don't bring Cortana back. The ending of Halo 4 was tragic and had so much more depth than we were expecting. If she came back, it'd be hard to take the games seriously.