As for specific plot details, I want to start things off with a bang. Writers are often fond of beginning with dark, cool prologues and epic battles; why not put our heroes square in the middle? And they don't even have to be involved in the same way - one could be a soldier enduring the onslaught, one could be a general sweating over his next move, another could be a civilian caught in the crossfire. One way or another, our heroes should be introduced in a fantastic opening battle to get our blood pumping.

I'd imagine that the battle would be a pyrrhic one so that the heroes would be forced to work together to survive the aftermath. What happens after that is up to you! ;)