Suggestions
1. Achievements and a timeline of Milestones.
2. A Banking app (balance, bills, transfers)
3. Kids who become real characters. At 18 they could become full NPCs with their own job, partner and home. Before that, you could do activities with them (read, play, help with school), and their personalities would mix their parents' traits.
4. Calendar events: festivals, concerts, markets, holidays and anniversaries, so the city has a rhythm beyond your own schedule.
5. More property: buy a second home or vacation house, and rent out rooms in the big residence.
6. A newsfeed or gossip system: what NPCs do shows up as city news, and your reputation changes how people treat you.