Perhaps the opposite - Cold Zones: the greater the daily average a zone has people adventuring within, the lower the overall modifier? The adventure part should be tied into the total number of slain monsters so places like East Commonlands does not decimate the younger crowds fighting in that zone while people buy and sell goods nearby. And perhaps an individual modifier so killing your first Oasis orc gains full experience, but after a couple hundred, a mere trickle instead.
Just some thoughts.
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