I don't know how long I need to keep repeating that the mod doesn't affect your projects, and since it doesn't, it's obviously built for version 3.1.
You never consider multiple perspectives, only seeing one. First, the class of cities is determined for all cities at once; second, even your idea of capturing a city with another guild is absurd, because you can capture a non-native city and also develop the guild; third, in the late game, you can obtain almost all the spells you need on the map; fourth, you would break the strategy on the global map, where the city is a key point not only for generating an army, but also for spells (heroes are RPGs, no, I haven't heard of that).
And as for 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 Inferno with Darkness + Light on the map, or an old Castle with Darkness + Light?