How long does it take to instill the idea that the "General balance" in your system is already being distorted by definition? If you are running away from changes to the General balance, then nothing should be changed in the system.
Too much fantasy (what I'm considering and what I would break), too few arguments. City classes help in supporting spell upgrades (if we're talking about the starting city), but their influence is much less (simply because the starting city is not the only object on the map). In the end, create a topic about city classes (but first, make sure it's technically feasible), and we'll talk about it there.
Added 1 minute ago
And regarding the fact that city classes apply to all cities, what difference does it make, from the point of view of obtaining spells, whether we encounter a new class-based Inferno with Darkness + Light on the map, or an old Castle with Darkness + Light?
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