Enhanced AI NPCs for player cities. This can be implemented as a feature mayors can use to make their cities feel more "alive", and gives an 'on demand' option for those who don't want to add it. Please also don't make it something that costs maintenance.
Suggestions for NPCs:
various wandering and/or static vendors, 'tourists', children (of various races.. because who doesn't wanna see little ewoks chasing each other around?), hobos, retired folks of various professions, visiting VIPs.