Hahaha, would be quite unrealistic, but I guess kinda cool to see
Speaking of wiping out hoods/cities, if you could realistically damage buildings etc, and then over time they become repaired... WOW. That's something that would blow me away.
And not to sound like 9-11 was awesome or anything, but being able to collapse skyscrapers and other buildings, that would be incredibly awesome, but for this reason. It would add to the sense of the game being alive, action all around. Pedestrians doing their own thing, catastrophes all around you, even if you're not nearby. Look up into the sky at a random moment and see smoke from a burning house, watch a line of people standing outside a club waiting to get in. If I could select the main focus for GTA IV, it would be this. A live world, expressing itself.