I know this is a bit outside of the scope of the game. But you said to post ideas ;)
I would love to set my co-player (who is a kid that is more about experiencing my town than building) as a tourist. Here is my idea:
A cozy survival mechanic. The guest has a bit of money and needs like hunger, relaxation, fun, toilet.
The guest needs to spend the money to fulfill the needs. If they a low they are doing a slow angry walk, angry smilies, if they are very fulfilled, then they walk with that happy dance animation.
Maybe there could be a wish system like an active quest that expires after a while.
Maybe keeping your needs up in a streak or fulfilling wishes could give happiness points that unlock something.
If the money is empty they have to do odd jobs to get some again.
That could be a new app for the mayor, where jobs can be assigned. From harvest x plants and put i storage to drive the unicorn tricycle to farming area 1.
Or things like watering plants or putting harvest and ore and wood into storage could give a fixed amount of money without a new app, depending on how in depth this would be implemented. This would be a fun way to show friends your city in online multiplayer too.