In the "Strategies and Tips" forum, a player named Noto complained how easy it is to raze cities. He is right. I would like to to see razing being more like pillaging.
Each ground unit in a captured city, but not yet occupied city (the option to keep the city is waived) may sack the city for a random amount of gold and drop in the city population of one citizen unit. This may continue over multiple turns.
I would also like to see a small (10%?) chance of capturing a worker while sacking, if the sacking civ is running slavery or vasselage civics, and a -1 relations modifier with other civs who are running more enlightened civics.

There was a Mod around that allowed something like what you are talking about. Basically, the larger the city & the fewer the units, the more time it took to raze the city completely. This, of course, gives you a reasonable length of time to send units to rescue that Holy City that the barbs are about to raze to the ground!

Aussie.

That is a good idea. I think each population unit should also be a kind of unit with an attack value to indicate civilian resistance to pillaging, so you may recieve some damage, or be forced out of the city by resistance.