Grando
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About books and the blue hat.
Let any hero be able to use spells only if they have the corresponding level of wisdom (and, in addition, let there be a "spellbook" purchased for warriors).
That is, control is certainly present in the book of air or the blue hat, but you can only use it if you have maxed out your wisdom (advanced for Town Portal in Castle).
Because these are all quite specific choices that you have to make in the game - and regularly between wisdom and logistics, or earth/air, tactics, offense.
And then this hero with high wisdom but low logistics cannot reach the opponent's city, or can reach it, but will not be as effective in melee combat. Etc.
And there are also random guys from dungeons who, unexpectedly, cannot teleport, immediately winning the game.
For starters, even this will noticeably slow down the pace that books and the blue hat provide.