The idea of finding treasure maps that display a very small screenshot of a level to dig up rewards sounds fun. Basically the map shows very small snapshot of an area and you'd have to find it based off of your knowledge of the overworld.
However while it'd be fun, it probably would't be an efficient way to make money. How would you get the maps? In chests? Then it'd just be better to just get gralats in the chest instead.
Could, however, randomly spawn a chest on the overworld at certain times that contains a map. This would be a much better way to encourage revisiting areas on a daily basis, as there would always be a new chest to find.