Sigh, Damien. You want mag/battlefield 3/WoW hybrid for the halo franchise? With fully fleshed out real AI, and fire teams? You don't know a thing about real world constraints do you? Theres this thing called memory, and theres a finite amount of it. There's a thing called processing power, and we don't have enough of it. There a thing called bandwidth, and your average in home system couldn't handle it. Not to mention the difficulty of creating true AI.
Videogames take millions to create. With all the manpower it would require, it would take more than a decade of production with a massive studio, assuming you throw all the impossible parts of it out. And when Microsoft operates on tight margins anyway, even attempting it would seriously put the company under.
Your idea is nice, but don't get your hopes up for anything that scale ever in your lifetime.
On a side note, 343 does have a lot of serious issues to fix, and they have not yet acknowledged these issues. On another side note, a good amount of 343i's employees were originally in bungie. In fact the director was the previous second in command of bungie i believe. Many people think Of them as separate, but they really are a lot more alike than a lot of people think.