Call of Duty 4: Modern Warfare is one of the best games on the 360. But no game's perfect, so here's some changes I would make for CoD6. I'll also throw in some ideas for the campaign.
American troops and our allies are stationed all over the world at any given moment. So how about some settings we're not used to seeing (southeast Asia, north Africa, South America, etc)? If the Middle East is again the focus, then possible settings include the caves of Afghanistan or a battle within a single building, such as a school or ampitheater.
Weather is a major factor in warfare. Hell, it has outright won or lost battles and even wars. Show us. Gusts of wind can displace tossed grenades and spread fires. Tropical rain can disrupt vision, slow movement, and make ways impassable. A sandstorm could trap enemies together
Soldiers don't just work and interact with fellow soldiers. Make civilians, combatants and non-combatants, a part of gameplay. In fact, you might include one level in which the player goes on a light-hearted hunting trip with a local civilian, to bind the player emotionally to the town or whatever he later defends and remind the player that war isn't always constant battles. I suggest hunting because that is a civilian activity based on the same basic gameplay (shooting), requiring less scripting and new assets.
Introduce friendly fire and civilian casualties. Both are a part of every war. If well implemented, this could get more emotional investment from the player.
Take your cues from Left 4 Dead. As I've said before, that game has the best representation of dependence and sacrifice of any game I've played to date. Brotherhood is a central theme in every branch of military, and cooperative gameplay is where that needs to shine.
Sacrifice is key. Enable the player to give to fellow players and companion NPCs in ways that hurt his own character and his own playtime. Regarding NPCs, I've written before on empathy. Regarding fellow players, consider the example in Left 4 Dead of one player telling others to leave him or her behind to save themselves... a request which means the person is no longer playing for a while, only watching and talking to the other players (sometimes feeding them valuable intel)... though obviously it's usually better if all players remain playing.
Every time the player goes to the "Select Class" screen, show a number by each class to indicate how many allies are currently playing each class. That will allow versatile players to fill in the gaps; help us see which roles are needed at any given moment.
Allow us to mute players without having to go to their gamercard screen.
Again, weather. If it was different from battle to battle (the same map), that could make for a fun dynamic.
You might be able to encourage a better social atmosphere by enabling players to vote each other up or down in a leadership ranking, separate from combat ranking. Real militaries do not promote soldiers purely for weapon skills and combat prowess, but also for exemplary social actions... like taking risks for other players, going back for injured comrades, helping others to improve, offering inspiration, and basically providing for the needs of others. How exactly to reward players for such behavior, I'm not sure. But it would definitely be worth it to promote sacrifice and philanthropy in the multiplayer community through tangible rewards, similar to a Purple Heart or Silver Star.
Again, sacrifice. Allow a player to give ammo to an ally, thereby depleting his own resources. Or allow a player to "cry for help", making his icon on allies' maps blink, to indicate he could use backup when he finds himself the only one guarding a flag in Domination or having run out of ammo.
What are your ideas?