Offline mode, I don't have access to internet sufficient enough for games, so this would be a welcome addition. I'd also want Marathon's Security Officer's armor and helm.
A gun that shoots Moa. Tap the trigger for a live one to appear in front of you, hold to shoot it at your enemies. Upgrades include flaming moa and sombrero moa.
I would add some kind of training place where you create your own battlefield. It would be like forge, but you could put down enemies and you could program them. Then you could go on the battlefield you made and fight the enemies. Also, you could download other people's creations also. I guess what I mean is, I like to have forge again, but with enemies that you can fight in the map that you made.