This wish is a point of contention among some of us. The D-Pad is just hanging out there, so why not assign some simple context sensitive commands? You know -- "follow," "hold position," "suppress," "cover me."
You could argue this turns "Halo" into a squad-based shooter. Nah, not really. It just provides some control over the Marines when they are around. Who's not tired of seeing them die senseless deaths or foil stealthy plans with their well- intentioned blundering? If you're not into the command menu, no sweat -- don't issue any commands, and the Marines just go about their business. It's only there if you want it.
Using the Elements
Everything is a weapon if you use your imagination. What if you could pick up boxes and canisters with the "X" button and chuck them at people with the right trigger? It's not going to do a ton of damage, but it would be a great option when someone is chasing you or you find yourself low on ammo. It could also apply to guns that are out of ammo. Once you run out the game could prompt you to "press 'X' to throw weapon."
Seriously, wouldn't you like to be able to lie down and fire in the prone position? While we're talking crazy, how about a roll? Bungie could pull this off in a few ways. Holding down the left thumbstick makes you crouch in both "Halo" games. Bungie could build on that. A simple click and release could put the player in a prone position. You would click the stick once more to stand up. Once the Chief is prone, forward and back on the thumbstick would make him crawl forward and back. Left and right would cause the Chief to roll in those respective directions. Think about it -- you could snipe from underneath a Warthog or just change the oil.