I think druids should suffer a % spellfailure when they are not in a wilderness area. Such as in any of the Sanctuary areas, they suffer a percentage of failure to their spells. For example a druid in the in-between or upper would have 15-20% spell failure, while say if a druid were to enter the machine itself they would get even greater spellfailure of 25-35%. I think this would reflect why druids do not stay in cities and towns, and make a mechanical incentive for druids to keep to outside of city areas as opposed to having no negative effects within the region. Much like how spellcasting has failure when casting underwater because you are interrupting the aspects of speech and gesture, druidic magic has failure in urban areas because you are interrupting the flow of ley.
An interesting suggestion, but not really in-line with the state of the gameworld at present.