= Mods that replace individual factions =

in theory, you can transform any castle into a city of mages, but the buildings, hero specializations, and city structures will remain from the original city.

No, the idea is simply to allow any city to take the place of any other.
The concept is as follows: there can be a maximum of 8 different factions on the map, but if the above-described idea is feasible, then these can be any (including cities from mods).
The map would be created very simply: the necessary mods would be placed in a folder, and then a map would be created using GSC or an editor, just like any other, but a special editor that works with mods would likely be needed.
We cannot directly add new factions, but we can create the illusion of adding them.