This was a planned feature of TA that looked like it was going to be seriously cool but fell through because of the complexities of it. From memory it was shelved with the idea of hopefully implementing it in GC3.
I don't want to know if this is true or not, it does sound like a very complex system to build and keep fresh, I'd much rather be surprised by finding it after playing my first game in GC3. I just wanted to put it back in every ones minds!
The community was always very, very good at creating our own full fledged stories out of our most epic games so I'm sure we'll be more than capable of entertaining each other either way!