They will be pretty full, I'm sure. But the one thing that will make cities feel more alive is not the NPCs in them, but the players themselves. So since they are using this megaserver technology you should be seeing a ton of players in the cities making them seem more alive.
I have a feeling that the cities will be instanced to that specific channel. I don't think they will go with just one instance of a city.
Cities, should be bustling with the amount of players and megaservers, I just hope the cities that aren't capitals are quite full as well.