Population control
What the title says. I want there to be a way to control population growth. The most important aspect of it that I want in the game is the ability to halt growth. The inability to do this in GC2 really drove me nuts, and I was very happy that Civ IV actually allowed you to do this. In Civ IV there was just a button that halted population growth with no strings attached, but enforcing population restrictions could cause some unrest (the amount could even depend on what the growth rate would've been - the faster the potential rate the more the unrest).
That's the one aspect of population control that I consider a requirement (with or without side effects). There are some other aspects to consider though. Like the ability to provide monetary incentives to have children, resulting in a faster population growth provided you have the food to fuel it. And even a similar incentive system to to encourage migration from one city to another.