Here's a few suggestions!
Game Mechanics Changes:
-New (possibly unlockable over time) skins!
-Save codes (get a code generated to encode your current stats in one session, paste it into another session, resume from where you were)
-Customizable town! (when some things are "built", you get to decide where they go. Different locations affect how well they do)
-Character development (some characters "remember" your decisions, and begin to like/dislike you over time)
-Resource changes (people die over time, possibly add "materials", "relations with other towns", "sketchiness" resources)
-Other Towns (other towns develop on their own: you can buy stocks in the town in separate windows. Stock values change every round)
New Encounter:
Aspiring Mayor: I wish to start my own town! Can I have some money and people to get started?
Y:
Aspiring Mayor: Thank you so much! I promise I'll be back!
-30 people, -500 gold, +10 happiness
After about 5-10 rounds:
Aspiring Mayor: My town is so successful! Here are some funds back!
Y:
Aspiring Mayor: Here you go!
-10 people, +800 gold
N:
O-Oh... Okay then.
-10 happiness
N:
B-but... Everyone was so excited...
-20 happiness
This is all I could think of for now... I hope it's good enough!