These just my Two Cents

Resource Management : Learn from AOW. You don't need a city to capture resource. Just put a unit on it, raise your flag and viola, you get the resource. But of course, you need to defend it, or else your opponents will take it over from you. This way, you just don't need to build an outpost on a resource.

City ruins : It from AOW too. There are cities ruin that you can take over to rebuild a city. But of course, it is not free. You need to clean the land from any monsters who rules the ruin before you can put your pioneer on it.

City is a production house. You need a city to build armies, let your heroes learn magic (from Heroes Might & Magic), and buy equipments. Of course, you also need a city as your government centre. But, once again, you don't need a city to take over a wild horse tiles or a lost library.

Learn from Total War, the bigger the city, the more gildar that lost from corruption / spoiler. The farther your cities from the capital, the more crime, corruption, and spoiler that you must handle.

Make the monsters more aggresive. So, expand too fast will make your city as monster's target practice. By this, you need to build slowly, build up defend, before ever think to spam more cities. Or your cities will become the monster's regular food source.

People are not passive. Rules are created to be broken. So as long as you don't put sentries on your cities, you'll get chaos reports from your cities. Like crime raise uncontrolable, frightening Citizens protests about the lack of defense, Annoying Quest of Children kidnapping monsters that cause panic to the whole city (and make the city doesn't produce anything until you solve it).

So just imagine if you get a lot of annoying problem because of lack of management to your cities. Then,  you'll think about to create a few healthy cities with smiling people who give you enough gildar to expand your empire / kingdom

What do you think?