In any normal or vanilla room, you can use the /map *mapcode* or /np *mapcode* to pick a map.
A window will appear asking you if you want to do it.
It will cost a certain amount of cheese or fraises, and like buying and customizing things in the shop, you have a choice of which currency to use.

For example, it could cost 15 cheese or 15 fraises. Maybe more, maybe less.
Either way, it'll cost a certain number of fraises or cheese.

In addition, perhaps the mayor would pay less cheese to change the map.
It's not much, but at least it'd make the mayor system a tiny bit more useful.

People would only be able to choose maps that are not deleted.

If this would lead to too much abuse, maybe make it only work for vanilla rooms where you could pick between in-rotation vanilla maps.