How about adding a weather system. U just take in all conditions u can think of: snow, rain, sunny, storming, scorching hot... and all conditions effect your exp/money gain. Like they both go faster when it snows and slow down when u have hot weather etc.
In addition u can add something like a "weather-control-device" to the map shop which u then can upgrade to make the weather change the way you like it easyier.
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