Wind of Change

The idea of having a separate level/slot for control is excellent. Setting a fixed price (increased, for example, 20 resources for Tier 3 creatures, 30 for Tier 4) would be a good approach. It shouldn't be removed from books/Pandora chests, as it provides an alternative way to obtain them.

The current random control in the Grand Mage has repeatedly shown that something needs to be changed. The variance can be too extreme, as Grand Mage 4 can be rebuilt 4-5 times (13 spells), and Grand Mage 5 can be rebuilt 5-7 times (6-8 spells). This results in a possible difference of 10-12 days!

This idea has been discussed many times in streams with players, and a significant number of them support it.

What is happening now in XL+U is absurd; the game has simply been reduced to repeatedly using the Grand Mage, and the difference in when control is obtained often decides the games. But a Grand Mage level 6 with control is complete nonsense. Players may nod and say yes, but imagine the implementation process? How should it look, at least from a design perspective? Should you build an additional floor in each city? Or add a secret slot with a custom spell to each level? Before suggesting something like this, it's better to think about it at least a little.

An interesting solution has been implemented in the Fresh mod, where the probability of obtaining spells has been changed.
1st attempt: normal chance, 2nd attempt: double chance, 3rd attempt: triple chance, and 4th attempt: 100%. It would have to be extremely unlucky to get both spells on the 4th attempt.