Ideas and suggestions.

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 them have 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 Armageddon's Blade).
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 then they won't be as effective in melee combat. Etc.
And there are also random guys from the Dungeon faction 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.