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Regarding the control and overall balance of magic, I already shared an idea, but it somehow went unnoticed. I'm not sure how atmospheric and classic it is; I'd like to hear Kot Hryu's comments on this.
The idea is this: when building a certain level of the Mage Guild, not only do the spells of that level appear inside, but a button also appears, which, when clicked, offers to buy a scroll with a spell of that level. The specific spell is not shown; it is chosen randomly. This is an alternative to rebuilding, but much more elegant and natural from a game logic perspective.
In general, this aspect seemed stupid to me even in the early 2000s, when I was just starting to play Heroes, that in the Mage Guild (a rather expensive building), a limited, fixed set of spells is given, which cannot be changed in any way. It's always incredibly annoying, for example, when the "Arrow" spell is not available at level 1. This especially applies to high-level spells; it's like, "we built and built, and finally we built it."
And from the point of view of atmosphere and game logic, it's the Mage Guild, wise wizards sit there, a magnificent building was erected for them at great expense so that they could meet there and share their wisdom with the heroes who come to them. And they, for the sake of appearances, drew a few spells, put them in a prominent place, and went to the tavern to drink. Like, "we've done our job, go away." They need to be kept busy, after all, or the old men will get bored, and all the wisdom of the ancestors will be lost.