Just yesterday I was watching the Halo 3: ODST Live Action Trailer and thought of what the game should have been. I honestly didn’t enjoy it, but I think Bungie was on to something. If they had taken the Reach approach where the character is YOU and improved on the whole “free roam” aspect from ODST, it could have been an awesome game. I’m also one of the fans who approve of a Halo game taking a different form (i.e. Halo Wars), so some people may not like what I’m about to say… but here we go!
The game revolves around you and the ship that you’re enlisted on. There are no ordered missions, except for maybe a few, and you’ll most likely have a different experience than the next guy who plays (at least the first time around).
As you roam around the ship, whether you’re exploring or looking for something, you trigger missions and events that shape your campaign. So if your friend was playing at the same time you are, but took a different route, he might end up playing a different mission.
It would be amazing to find yourself walking down a corridor and all the sudden an alarm sounds and everyone starts going crazy. As an ODST, you and your fellow soldiers would be ordered to your pods while the rest of the ship’s crew take their positions. After that, assuming this is a ground operation, you’d probably be shot down to whatever planet lies below, kill a few bad guys, and do more mission stuff… But maybe a Covenant ship sends a boarding party to your ship, well in that case you might report elsewhere and do something entirely different… but you get the idea.
After a few events have passed, your luck runs out. To keep the game from getting stale, something drastic has to happen… Your ship gets damaged to the point where you’re forced to bail out on another planet nearby. This is where the rest of your free roam campaign begins. This time, however, it’s a whole new ball-game and you’re no longer confined to a ship. You are now a lone ODST on a planet you’ve never visited and need to move. By now you’ve encountered a few enemy squads and have traveled far from where you’ve landed. Eventually though you find familiar faces and need to come up with a plan. Your squad decides that looking for survivors and maybe a way out is the way to go.
In the end… who knows. Maybe you (not your character) find out as you were bailing out of your ship that you were enlisted on the Spirit of Fire. It is now destroyed and the remaining survivors, including you, are low on morale. As numbers decrease the whole situation turns grim. The last person to die was one of your good friends and now it’s just you again. I could only explain it as being similar to the way Reach ended when you suddenly realized you were alone.
If this game was to ever happen and I was leading it, I’d say you have a few choices as to how you wanna end the game though. One of which would probably be an amazing ending like Noble Six’s. The other choices I have not thought of… mainly because I don’t have to and 343 will never ask me to make this game… But I can dream!
If you read the whole thing then thanks, provide some feedback below. I hate wasting grade A material like this. 
Oh, and this is more of an example, not the way the game should really go. I hope I got the message through though, it’s a hard idea to explain when there’s so much to include.