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Why, I wonder, are they mixing two completely different ideas?
To introduce, so to speak, mages into the game and allow players to use 4th-level magic without restrictions. The second doesn't follow from the first at all. Mages won't gain much weight in the game. Instead, there will be an excellent chance to take advantage of the situation. Free a 10-20 level mage from prison (I, for example, found 10-level mages even on the first map), give him a few fast units, and cause chaos in the enemy's surroundings. With the same success, you could allow diplomacy, but with a ban on starting with diplomatic heroes. This also creates a chance to find such a hero in the tavern and go for it. Moreover, the chance for both opponents is the SAME. And the rules will become even simpler.
To strengthen the role of mages in the game, I think it's necessary to change the template, not to reinvent the wheel.