You can add NPC babies, which you can adopt and they grow little by little as you take care of them, being able to save the baby's process until it reaches adulthood, when it is you can already give the "independent" option. You can also do this with pets, such as dogs or cats. By caring, I mean feeding them, sleeping, taking them to school, etc. You could add the option of being pregnant so that you can take care of the baby from the first moment.

You can also add traffic rules, such as penalties for jumping a traffic light, stop sign and speed limit. As you can also add intermittents and a police work along with NPC cars that we can penalise these NPCs.